I just added a password on my computer (Microsoft Edge) and Lastpass sync on my Android device (Galaxy S21), however, the log in was only partially synced - only the URL, password and category - no username. So now, Lastpass shows the username for this log in in my browser vault, but not on my Android vault. I've tried logging out of Lastpass on my Android device and logging back in, but nothing is updated.
Did you search for the site name in case there are two entries?
Otherwise, the desktop app may not have polled the server. Try a force refresh on the desktop first (LastPass Icon > Advanced > Refresh Sites), and then on the Android app by either swiping your vault list down from the top, or log out/back in.
I did do a search, which is how I access the log in on my Android - only the 1 entry.
Unfortunately, I ended up manually updating the log in on Android to match the desktop entry, so I can't test your solution. I'll try it if this happens again.