Working with the V12 Repo Client, we have found that the thumbnail pane shows up blank if it is un-docked.  It looks like this:

If you resize it or dock it, it populates correctly. However, if it remains undocked, it stays blank if you close the document and open another.  User reactions to this range from confusion to panic.  If you have them dock the pane, then it acts normally every time. If you can get it to dock, which is a different issue. This was true on a straight Windows machine and also on an Azure Virtual Machine.