Lots of threads missing in Users.threads.list()

Question

I am retrieving threads from my GMail account using the Users.threads.list method, with the query "in:sent newer_than:1y". The first page of results (100 threads) looks OK, with threads from Dec 2014 and Jan 2015. However, the second page starts at June 2014, and goes back to May 2014 etc. It looks like whole pages of threads are missing for the months of July-November 2014.

I have reproduced this using the in-page API tester at https://developers.google.com/gmail/api/v1/reference/users/threads/list.

Everything looks fine in GMail's web interface.

I'm pretty sure this worked yesterday, i.e. I was able to retrieve all threads / emails of 2014.