This one is a little obscure… If you turn on Show Community Header under General -> Top Toolbar, then switch to a section like “All” using the left drawer, then switch to another section like “Subscribed” or “Local” by tapping on the top header (not in the left drawer), the top header will disappear when you are at the top of the feed, and reappear as soon as you start to scroll. Scrolling back to the top makes it disappear again.

It doesn’t seem to affect community headers, only the views like All, Local and Subscribed. Turning off the Show Community Header option prevents this issue. Switching sections only from the left drawer also prevents this issue.