This indicates that we're incorrectly reconstructing pins when the REINST classes are still around.
The root cause is actually due to a cyclic dependency with external actors when using World Partition. Adding actor literals to the level Blueprint will reference the Blueprint class of the actors. In turn, the external actors have a reference to the level.
When a level is loaded, we end up in a scenario where the level Blueprint is compiled prior to the call to ReplaceInstancesOfClass that repairs the BP classes to use the newly compiled UClass. Ideally, we should defer the level Blueprint's compilation.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-184081 in the post.