It was a saved password as described above. Deleting it seems to have resolved the problem.
Thanks for the help.
LastPass has responded to these complaints multiple times. The browser's password manager has the master password saved and is overwriting the field. The solution is to go into that database, and delete anything related to LastPass. If you use Google Chrome or Microsoft Edge, there will probably be a key icon in the URL bar. Click that, and delete the entry.
Alternatively, make sure you are using the LastPass icon in your toolbar to access your vault, not any bookmarked links.
I have the exact same problem ! Why is there no response from Lastpass support!