PIE registers 2 Default Brushes in the blank level. It's expected behavior to have 1 Default Brush, which is the case in other play modes, but PIE registers 2.
This is an issue for our game because I am using an Actor's index in the level's Actor list (ULevel::Actors) to create initial seeds for random number generators (with FRandomStream), to ensure that the level plays deterministically every time it's played, but to ensure that each actor of the same class generates different random numbers. My assumption was that the level's actor list would always initialise the same way, as it's essentially created by the level designer. Apparently this isn't the case.
Results: PIE registers 2 Default Brushes in the blank level
Expected: For only 1 Default Brush to be registered in any playmode
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-46778 in the post.