Hello,
Avast opens the UI only once for every user. It is part of the on-boarding flow. If you install AV it affects all the users on the Mac and they should know about it.
I guess that Guest accounts delete all their data after logout, thats why Avast does not remember that it had already shown the on-boarding. It is a good idea to not show this at all for guest accounts. We will take a look whether we can figure out if we run under Guest Account or not and not show it at all in such a case.
But it will take some time to get it public as we have just released new version 13.12. One think that comes to my mind is a workaround. I have it prepared, but can't upload it here. If you want me to provide it, please contact me via e-mail. (jakub.bednar at avast.com)
Best regards,
Jakub