Keyboard Layout after startup reverts to "ABC - Extended" when using FileVault
I wrote an article called How to fix macOS startup keyboard layout not persisting about this very problem.

I never figured out exactly what the problem was, but the ABC Extended keyboard layout is the default fallback when no other keyboard layout is configured. The pre-boot environment stores its settings separately from the rest of the system.

Sometimes these settings go out of sync, or the system fails to save settings to the pre-boot environment. You’re more likely to run into this when using FileVault disk-encryption because of the increased complexity in this setup.

I lay out 11 steps in the above article that gets macOS to regenerate the settings file in the pre-boot environment. With the help of my readers, I’ve identified at least three different problems that these steps can resolve. It’s easier to just diligently follow all the steps exactly than to first try to identify the exact issue and then try to fix it.