I believe this is the same problem I have:
https://vborg.vbsupport.ru/showthread.php?t=155455
But still no definitive solution to date.
From the tests I did last time, it seems to be a problem with session. I suspect it wasn't able to save because the session timed-out or somehow became invalid.
During my tests, if user logs in with "remember me" checked there seems to be no problem saving the scores. This could be why the problem appears "randomly".