I have noticed that this does actually happen after I lock my screen as well.
I noticed one more possibly important fact. If I type characters (any number of characters) of the password in GDM login screen and then backspace to delete them all... then type in the password and press enter... it works.
So assume my password is 'MyPass!' (hypothetically), I type 'M' then backspace to delete that character then proceed to type 'MyPass!' it will then allow me to log in on the first attempt. Or... I type 'MyPass!' on first attempt (no backspace trick) and it fails... then type 'MyPass!' on second attempt... it works then too.
This seems very much like the first character is somehow entered wrong, but then just the act of deleting it and retyping it, or entering the password a second time it is entered correctly. Maybe libinput issue or similar?
Some extra info...
I have noticed that this does actually happen after I lock my screen as well.
I noticed one more possibly important fact. If I type characters (any number of characters) of the password in GDM login screen and then backspace to delete them all... then type in the password and press enter... it works.
So assume my password is 'MyPass!' (hypothetically), I type 'M' then backspace to delete that character then proceed to type 'MyPass!' it will then allow me to log in on the first attempt. Or... I type 'MyPass!' on first attempt (no backspace trick) and it fails... then type 'MyPass!' on second attempt... it works then too.
This seems very much like the first character is somehow entered wrong, but then just the act of deleting it and retyping it, or entering the password a second time it is entered correctly. Maybe libinput issue or similar?