Physics Asset falls through floor during simulation in Physics Asset Tool

When I press the Simulate button in the Physics Asset Tool, Physics Assets fall through floor. This was not a problem before. I haven’t switched UE versions ( version is 4.14 ).

This is not a problem in-game: If I place my physics asset directly in a map and play, it behaves normally. This is only a problem in the Physics Asset Tool.

Any ideas what’s going on?