Firefox browser (129.01) on Windows 10.
Note that I have to type Space-Backspace in both the User ID and Password fields to get it to work. This behavior is consistent (not just a one-time case).
That is a security function of the target web site. The person who programmed it is looking for actual keystrokes rather than an app filling in the password. The following is the workaround that seems to work 90% of the time for me. When your password is filled in by LastPass, add a space after it, then immediately delete that space. You have then satisfied the algorithm that is looking for a key press. Another workaround is to press the TAB and SHIFT-TAB keys. You may also need to try my workaround with your user name field.