Mail notification shows the outline of a box where the number of unread messages is supposed to show in the notification area icon. This seems to be sporadic -- sometimes I see the number of messages, sometimes I see a box instead. Current Rawhide versions of everything.
I think this has to do with auto-hide. When my bottom bar hides and then pops back up when I move my mouse to the bottom of the screen, the number is wiped and doesn't redraw.
Upstream say this will be fixed in the upcoming 5.1