Azure Active Directory B2C custom error page is ig

We have a custom Azure AD B2C Sign-in/Sign-up policy in place (SAML-based, the default policy doesn't do what we need).

We're able to customize the page UI as expected, via a content definition like:

<ContentDefinition Id="api.localaccountsignin">
  <LoadUri>https://example.com/SAMLSignIn.html</LoadUri>
  <RecoveryUri>~/common/default_page_error.html</RecoveryUri>
  <DataUri>urn:com:microsoft:aad:b2c:elements:selfasserted:1.1.0</DataUri>
  <Metadata>
    <Item Key="DisplayName">Local account sign in page</Item>
  </Metadata>
</ContentDefinition>

But attempting to customize the error page, via:

<ContentDefinition Id="api.error">
  <LoadUri>https://example.com/SAMLErrorPage.html</LoadUri>
  <RecoveryUri>~/common/default_page_error.html</RecoveryUri>
  <DataUri>urn:com:microsoft:aad:b2c:elements:globalexception:1.1.0</DataUri>
  <Metadata>    
    <Item Key="DisplayName">Error page</Item>
  </Metadata>
</ContentDefinition>

changes nothing. The SAMLSignIn.html page is still used (albeit with the contents of whatever signin error occurred). This happens whether we test with invalid user names, bad passwords, whatever.

Even setting

<Item Key="RaiseErrorIfClaimsPrincipalDoesNotExist">true</Item>

in the associated <ValidationTechnicalProfile> doesn't force the error.

The user journey as seen in ApplicationInsights shows only a Web.TPEngine.Providers.BadArgumentRetryNeededException; no other errors or exceptions are logged.

How can we cause a custom error page to be used on login failure (for whatever reason)?

The documentation, such as it is, seems to imply we're doing the right thing (as does How to Create a Custom Error Page in Azure AD B2C | Custom Policies). I can't find any suggestion that we need to specify custom error handling in the UserJourney, nor any means by which we could do so if we wanted.

To summarize:

You are using local account sign-in with SAML issuer. This makes sign-in process happen on Azure AD B2C sign-in page. The error messages you see are just regular validation error messages. This all happens on the sign-in page, not the error page. Error content is only used by the engine when an unhandled exception breaks the execution of the user journey. Which is not the case with a regular sign-in.

Your option to customize validation errors are limited to using pure CSS on the sign-in page.

A really complex customization is demonstrated here

The document describing UI customization in general, is here

And UI customization using custom policy here

At your best effort you will come with some nice error messages like these: