This feels like it's probably a stupid oversight chain like, keyboard layouts are user-specific data, so they're not decrypted before first unlock/set globally because the machine might have multiple users with different keyboard layouts.
I've never had a reason to try it, but there's also a remark that 99% of the Macs sold probably don't need to change the system-wide keyboard layout defaults, either...
[1] - https://heylon.ca/how-to-permanently-switch-default-keyboard...
[2] - https://support.apple.com/guide/mac-help/change-the-system-l...