Time seems to cause this more than anything else and I cannot replicate at-will. (I have actually been trying to catch the condition that causes this for a few months, but finally gave up today.)

Backing out from other communities will sometimes trigger this, but not always. I have only noticed this on c/all. If it seems I have been in one community for a "bit of time" and have locked/unlocked my phone a couple of times, the chance of this occuring seems higher. (That theory is kinda bullshit, but it's something)

Reloading the feed from the main menu clears the condition. (I haven't checked if a standard feed refresh works after the last couple of updates. It usually hasn't worked.)