Obviously the screen should not come up again after a successful login. I suggest that you get a log file for the developers to look at.
Restart your Volumio system and use it normally. When you see an unexpected login screen, do these instructions to create a log file.
https://volumio.github.io/docs/User_Manual/Sending_logs_for_troubleshooting.html
It will give you a URL to the log file, and you should post the URL here in this thread.