Obviously having a password field with a relevantly small maxlength is already a strange choice, but some websites do this on their account registration forms, their password reset or recovery forms, and their login forms.

I'm not sure what the best handling of maxlength is for a password manager, but at a minimum drawing the user's attention to it and highlighting when we have saved or autofilled a password that does not fit in that maxlength would be good.

It creates a small security problem too, for users that use memorable (low entropy per character) passwords, they might prefer a different password mechanism where a maxlength is imposed.

I've also seen cases where the autofill has exceeded the maxlength, somehow it can bypass that, which can lead to a whole other level of confusion. Probably a bug? Again, notifying the user of the issue should be the baseline.