Or logging in manually (known password)/autofill by another tool (browser) on a locked vault.
When the extension is locked, everything is encrypted including the URL, so it does not know you have an entry for this site. Triggering auto-fill (shortcut/context-menu) will prompt to log-in and then auto-fill. If the extension finds an entry that matches, it will not prompt to save/update that entry.
No not at all, but I have seen cases where people still entered their known password (not best practice as it’s not generated and likely not random/complex). Or still having the password saved in the browser’s password manager and use that to auto-fill. All of them with a locked vault.
I’m glad to hear you were able to figure it out. Yes, indeed the username is one part of the equality check.