Setup described in UDN post:
parent tree: root - selector - run behavior task
subtree: root - sequence - custom BP task with just print string
repro: print string message (engine tick) is showing up less often during PIE than standalone game
Only difference I can think of is sync with behavior tree debugger, maybe it's slower than expected during PIE.
1. create two behavior trees as in description / UDN post
2. start any map with AI using parent tree, PIE or simulate
3. on screen prints are noticeably slower after ~5s if behavior tree editor window is opened
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-29029 in the post.