API call users.list(exclude_external=True) filters out wrong users #3113

I recently updated to 17.8 and the behavior of filtering internal/external users does not work properly.

We are on self-managed GitLab free EE. Newly registered users are automatically flagged as external, except when their email matches a specific regex (admin settings > account and limit).

Prior to the update, new users got the attribute external=true and people matching the regex got external=false. Now after the update, people matching the regex get external=null. I'm not sure when that behavior changed.

The problem now is, that an API call like gl.users.list(exclude_external=True) for some reason filters out accounts both with external=true and external=null, basically rendering it useless. Older accounts where the flag is still external=false work as expected.

Any ideas what's going on?