Why is matplotlib basemap not plotting the colours

The code below is supposed to colour all the states of Vietnam:

import pandas as pd
import matplotlib.pyplot as plt
from mpl_toolkits.basemap import Basemap

fig, ax = plt.subplots(figsize=(10,20))

# create the map
map = Basemap(resolution='l', # c, l, i, h, f or None
            projection='merc',
            lat_0=15.95, lon_0=105.85,
            llcrnrlon=102., llcrnrlat= 8.31, urcrnrlon=109.69, urcrnrlat=23.61)



# load the shapefile, use the name 'states'
map.readshapefile(r'path\to\gadm36_VNM_1', name='states', drawbounds=True)
# shapefile downloaded from http://www.gadm.org/  



# collect the state names from the shapefile attributes so we can
# look up the shape obect for a state by it's name
state_names = []
for shape_dict in map.states_info:
    state_names.append(shape_dict['VARNAME_1'])

ax = plt.gca() # get current axes instance



# NOR, CEN, SOU and MEK are some subdivisions I have created for the states of Vietnam 

NOR = ['Lai Chau',
'Lao Cai',
'Ha Giang',
'Cao Bang',
'Dien Bien',
'Son La',
'Yen Bai',
'Tuyen Quang',
'Bac Kan',
'Lang Son',
'Thai Nguyen',
'Phu Tho',
'Vinh Phuc',
'Hoa Binh',
'Ha Noi',
'Bac Ninh',
'Hai Duong',
'Hung Yen',
'Ha Nam',
'Quang Ninh',
'Hai Phong',
'Thai Binh',
'Nam Dinh',
'Bac Giang',
'Ninh Binh']



CEN = ['Thanh Hoa',
      'Nghe An',
      'Ha Tinh',
      'Quang Binh',
      'Quang Tri',
      'Thua Thien Hue',
      'Da Nang']



SOU = ['Quang Nam',
      'Kon Tum',
      'Quang Ngai',
      'Gia Lai',
      'Binh Dinh',
      'Dak Lak',
      'Phu Yen',
      'Khanh Hoa',
      'Dak Nong',
      'Lam Dong',
      'Ninh Thuan']




MEK = ['Binh Phuoc',
      'Dong Nai',
      'Binh Thuan',
      'Tay Ninh',
      'Binh Duong',
      'Dong Nai',
      'Ba Ria - Vung Tau',
      'Ho Chi Minh',
      'Long An',
      'An Giang',
      'Dong Thap',
      'Tien Giang',
      'Kien Giang',
      'Can Tho',
      'Vinh Long',
      'Ben Tre',
      'Hau Giang',
      'Tra Vinh',
      'Soc Trang',
      'Bac Lieu',
      'Ca Mau']



# Define the colours to be used to colour the states

from matplotlib import cm
from numpy import linspace

start = 0.5
stop = 1.0
number_of_lines= 4
cm_subsection = linspace(start, stop, number_of_lines)

cm_subsection[0] = cm_subsection[0]*4
cm_subsection[1] = cm_subsection[1]*0.6
cm_subsection[2] = cm_subsection[2]*0.8
cm_subsection[3] = cm_subsection[3]*0.1

colors = [ cm.Blues(x) for x in cm_subsection ]


for state in NOR:
    seg = map.states[state_names.index(state)]
    poly = Polygon(seg, facecolor=colors[0], edgecolor=colors[0])
    ax.add_patch(poly)

for state in CEN:
    seg = map.states[state_names.index(state)]
    poly = Polygon(seg, facecolor=colors[1], edgecolor=colors[1])
    ax.add_patch(poly)

for state in SOU:
    seg = map.states[state_names.index(state)]
    poly = Polygon(seg, facecolor=colors[2], edgecolor=colors[2])
    ax.add_patch(poly)

for state in MEK:
    seg = map.states[state_names.index(state)]
    poly = Polygon(seg, facecolor=colors[3], edgecolor=colors[3])
    ax.add_patch(poly)





import matplotlib.patches as mpatches

NOR_patch = mpatches.Patch(color=colors[0], label='Rate: 34.85%')
CEN_patch = mpatches.Patch(color=colors[1], label='Rate: 25.61%')
SOU_patch = mpatches.Patch(color=colors[2], label='Rate: 32.66%')
MEK_patch = mpatches.Patch(color=colors[3], label='Rate: 20.02%')
plt.legend(handles=[NOR_patch, CEN_patch, SOU_patch, MEK_patch])
plt.show()

But this produces the map below, where some of the states are not coloured even though they are present in the state names and in the subdivisions:

In fact, if I try to colour a state whose name is not present in the lists, it throws an error:

MEK.append('ABCDE')

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-619-a89da62a0831> in <module>()
    134 
    135 for state in MEK:
--> 136     seg = map.states[state_names.index(state)]
    137     poly = Polygon(seg, facecolor=colors[3], edgecolor=colors[3])
    138     ax.add_patch(poly)

ValueError: 'ABCDE' is not in list

So, clearly the states that are not colored are present in the list, as I'm not getting any error. So, what's going on?

EDIT: It just struck me that almost all the states that aren’t coloured share at least some part of their border with sea/ocean in the real world. The 6 exceptions are highlighted in red below:

Now that’s very interesting! Could it have anything to do with the issue? If yes, what? And why? And why do those 6 exceptions exist?

EDIT 2: I get similar results when drawing the map of Philippines:

In shapefiles, a country/province/whatever, may be broken down into several line segments. Why that is, I don't know, but in order to plot the shape correctly, you need to use all the necessary segments. Actually, in the Basemap documentation for shapefiles there is an example under 'filling polygons', how to do this correctly. I adapted their example to your use case. It's probably not the most optimal solution, but it seems to work.

import pandas as pd
import matplotlib.pyplot as plt
from mpl_toolkits.basemap import Basemap
from matplotlib import patches as mpatches
from matplotlib import cm
from numpy import linspace
import matplotlib.patches as mpatches
from matplotlib.collections import PatchCollection

fig, ax = plt.subplots(figsize=(4,8))

# create the map
map = Basemap(resolution='l', # c, l, i, h, f or None
            projection='merc',
            lat_0=15.95, lon_0=105.85,
            llcrnrlon=102., llcrnrlat= 8.31, urcrnrlon=109.69, urcrnrlat=23.61)

# load the shapefile, use the name 'states'
map.readshapefile(r'shapefiles/gadm36_VNM_1', name='states', drawbounds=True)
# shapefile downloaded from http://www.gadm.org/  

# collect the state names from the shapefile attributes so we can
# look up the shape obect for a state by it's name
state_names = []
for shape_dict in map.states_info:
    state_names.append(shape_dict['VARNAME_1'])

ax = plt.gca() # get current axes instance

# NOR, CEN, SOU and MEK are some subdivisions I have created for the states of Vietnam     
NOR = ['Lai Chau',
'Lao Cai',
'Ha Giang',
'Cao Bang',
'Dien Bien',
'Son La',
'Yen Bai',
'Tuyen Quang',
'Bac Kan',
'Lang Son',
'Thai Nguyen',
'Phu Tho',
'Vinh Phuc',
'Hoa Binh',
'Ha Noi',
'Bac Ninh',
'Hai Duong',
'Hung Yen',
'Ha Nam',
'Quang Ninh',
'Hai Phong',
'Thai Binh',
'Nam Dinh',
'Bac Giang',
'Ninh Binh']

CEN = ['Thanh Hoa',
      'Nghe An',
      'Ha Tinh',
      'Quang Binh',
      'Quang Tri',
      'Thua Thien Hue',
      'Da Nang']

SOU = ['Quang Nam',
      'Kon Tum',
      'Quang Ngai',
      'Gia Lai',
      'Binh Dinh',
      'Dak Lak',
      'Phu Yen',
      'Khanh Hoa',
      'Dak Nong',
      'Lam Dong',
      'Ninh Thuan']

MEK = ['Binh Phuoc',
      'Dong Nai',
      'Binh Thuan',
      'Tay Ninh',
      'Binh Duong',
      'Dong Nai',
      'Ba Ria - Vung Tau',
      'Ho Chi Minh',
      'Long An',
      'An Giang',
      'Dong Thap',
      'Tien Giang',
      'Kien Giang',
      'Can Tho',
      'Vinh Long',
      'Ben Tre',
      'Hau Giang',
      'Tra Vinh',
      'Soc Trang',
      'Bac Lieu',
      'Ca Mau']

# Define the colours to be used to colour the states    
start = 0.5
stop = 1.0
number_of_lines= 4
cm_subsection = linspace(start, stop, number_of_lines)

cm_subsection[0] = cm_subsection[0]*4
cm_subsection[1] = cm_subsection[1]*0.6
cm_subsection[2] = cm_subsection[2]*0.8
cm_subsection[3] = cm_subsection[3]*0.1

colors = [ cm.Blues(x) for x in cm_subsection ]

##collecting the line segments for the provinces:
patches = {state: [] for state in NOR+CEN+SOU+MEK}    
for info, shape in zip(map.states_info, map.states):
    for state in NOR+CEN+SOU+MEK:
        if info['VARNAME_1'] == state:
            patches[state].append(mpatches.Polygon(
                shape, True,
            ))

##coloring the the provinces by group:
for state in NOR:
    ax.add_collection(PatchCollection(
        patches[state], facecolor = colors[0], edgecolor=colors[0]
    ))

for state in CEN:
    ax.add_collection(PatchCollection(
        patches[state], facecolor = colors[1], edgecolor=colors[1]
    ))

for state in SOU:
    ax.add_collection(PatchCollection(
        patches[state], facecolor = colors[2], edgecolor=colors[2]
    ))

for state in MEK:
    ax.add_collection(PatchCollection(
        patches[state], facecolor = colors[3], edgecolor=colors[3]
    ))

NOR_patch = mpatches.Patch(color=colors[0], label='Rate: 34.85%')
CEN_patch = mpatches.Patch(color=colors[1], label='Rate: 25.61%')
SOU_patch = mpatches.Patch(color=colors[2], label='Rate: 32.66%')
MEK_patch = mpatches.Patch(color=colors[3], label='Rate: 20.02%')
plt.legend(handles=[NOR_patch, CEN_patch, SOU_patch, MEK_patch])
plt.show()

The result looks as expected: