Hi Peter,
I wish I had an answer for this, but I see no one else has given you one. I'm having similar issues, and it's REALLY frustrating. I'm on an online game and it disconnects me from the game even if the interruption is mere seconds. Like you, I have an excellent connection to the net, and it's not the game disconnecting me... it's Avast reconnecting. It also might be after 2 hrs, might be after 10 minutes, or it might let me stay online and idle for days on end. I don't see any relation between Win 10/Win 11 install, as that was quite some time ago for me.
I just wanted you to know you're not the only one. Hopefully, someone will get this squared away. Thanks for opening the post!