Pokemon GO Gym Disappeared: What the Bug is and How to Fix it

Pokémon GO gyms have begun disappearing since the introduction of the latest update to the game. Gyms have disappeared in the past, but the reason for the latest rash of disappearances is different. Here's what you need to know.
I'm really looking forward to this but since the last update, I have to restart the app every 10 minutes because the gyms disappear! Hard to raid when the gym isn't there!
— Bob Gosh (@bobgosh78) January 24, 2020
Pokemon GO Gym Disappeared: What the Bug is and How to Fix it
Pokémon GO gyms are tied to notable locations in the real world. That real-world tie has gotten the game into trouble, as developer Niantic did not screen its list of locations before making them in-game landmarks. Many crossed into private property, or encouraged trespassing.
The first round of gym disappearances was tied to complaints around players crossing into private property, but this latest round is unrelated to that issue.
Since the introduction of the latest game update, players have reported gyms disappearing, only to reappear once they restart their game. The exact reason for the bug is unknown, and restarting is the only widely available solution. A more permanent fix will require a patch from Niantic.