I had similar issue.
For last 5 years I've been using macbooks 2019 and never had any issue with entering my password, I have always used Turkish Q keyboard and same password for years. After installing Tahoe 26.1 today, my password did not accepted.
I tried multiple times - the system locked me out for 15 minutes. After that (luckily), I thought about maybe the keyboard settings turned into US- UK settings for some reason - and since my password has Turkish letter ''i'' in it, I remembered for turkish keyboard it would be dotless ı - and it worked!
I managed to log in only because I had prior experience with an English keyboard layout and guessed the key mapping difference. Many users may not be able to resolve this. Literally, I entered wrong password purposely and solved the issue - even though the top corner keyboard shows Turkish Q - the system thinks it is not because it is accepting ''ı'' instead of turkish placements of ''i''
I tried again, and unfortunately the problem is still exists, after the update the system still thinks I am using english keyboard. I reported this to Apple, they need to ensure that the login screen preserves and applies the user’s stored keyboard layout settings instead of defaulting to English.