First letter sometimes is cutted

I do not know why, maybe you can try with 'emacs -Q'

FWIW, M-x vertico-posframe-cleanup seems to solve this issue (not sure what triggers it just yet, though).

I came across this issue sometimes. But can't find a certain way to reproduce it.

Me too. Meet this problem sometimes. Can't find out the pattern of reason.
You can see, not just the candidate hidden prefix part, also the prompt hidden some part too.

After checking source code. My suspect points is posframe-show's option :lines-truncate.

vertico-posframe/vertico-posframe.el

Line 288 in 7da6d64

:lines-truncate (buffer-local-value 'vertico-posframe-truncate-lines buffer)

And in posframe.el library soruce code:

https://github.com/tumashu/posframe/blob/3b97dc180b03498103cfcc7f44e64150df440bf0/posframe.el#L443

https://github.com/tumashu/posframe/blob/3b97dc180b03498103cfcc7f44e64150df440bf0/posframe.el#L628

The truncate-lines is from Emacs built-in support, I guess maybe Emacs built-in truncate-lines has issue.

This is what I can find. Hope someone can locate the problem.