Hitting check(UploadInfo.InstanceCustomDataCount * InstanceCount == UploadInfo.InstanceCustomData.Num()) and also seeing incorrect isntance order after removing instances from HISMs.
Additionally crash will occur when removing all instances through HierarchicalInstanceStaticMeshComponent::RemoveInstance().
Create an actor, add an HISMC to it
Set the HISMC to have NumCustomDataFloats > 0
Set a mesh on the HISMC
Have the player pawn trace to a location and on click, call HISMC>AddInstance(HitResult.Location)
Set up the right click to call HISMC>RemoveInstance(HitResult.Item)
Place several instances and then delete some.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-180750 in the post.