Separate out GameApplication onCreate additions always needed vs non-PSO processes by adding <GameApplicationOnCreateAdditionsAlways> for current case and moving <GameApplicationOnCreateAdditions> to non-PSO check.
Verify not seeing any initializations from each PSO process spawned (check logging, etc).
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-362487 in the post.