I have a couple more information for this. It's not limited to Groups Activity.

I think I know how it happens (had no time to test it yet):

* Have a long list of Freinds who are online or a lot of open group instances

* Scroll down in your list all the way

* close menu

* wait some time until many left VRChat or many group instances got closed

* open up your menu again.

Workaround: Scrolling (if you can scroll) up fixes it