It might be an issue with the right/left click auto fill on the latest LastPass browser extension. I had a similar issue where it "filled" the email/logon for the site, but not the password. Then, it would generate a new password. What I actually wanted to do was "use saved password" the last and least-obvious choice. LastPass did the without warning me, or probably not warning anyone. Seems to me like they've been making undocumented changes to the browser plugins lately.
If you are using LastPass, just turn off your browser's password manager. You don't need it and LastPass is better and more secure. I don't think any of the browser-included password managers offer the security of LastPass, IMHO. Why double your exposure and/or "attack surface"?
You have a saved password associated with LastPass in your browser's internal password manager. It's causing the browser to overwrite the field. Go into that password list and delete the entry.