If you have a sub level with "Initially Loaded" set to true and "Initially Visible" set to false it will crash after exiting PIE.
This issue was reported and tested in 4.20.1 (CL-4233996). It has been reproduced in Main 4.21 (CL-4234136). This is a regression from 4.19.2 (CL-4033788).
Results: The Editor crashes
Expected: PIE exits successfully
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-62352 in the post.