1. View any world in the worlds menu.
  2. Click the world author's name or group's name, if a group public.
  3. Click the back arrow.
The instance list will clear and not come back until you back out of the world in the menu and come back in. The thumbnail also clears and goes black.
This is a pretty big annoyance since that's the only way to know what a group's info or rules are before joining the instance, so if you're looking through multiple group instances, it's a lot of extra clicks with this bug.