I am having the same issue - very frustrating!
And there doesn't seem to be a way to talk to support about it.
I found this yesterday and it seems to have resolved mine, give it a go!
https://support.lastpass.com/s/question/0D5TP000006AJW80AO/last-pass-keeps-changing-my-passwords-from-what-is-saved-to-my-master-password-this-only-happens-when-i-enter-edit-mode-on-a-saved-password-and-click-on-the-eye-to-reveal-the-saved-password?language=en_US
I think you only get to talk to support on a paid plan, which is a bit rubbish