Check UDN for repro tree setup, it should be visible on something like:
root selector
(decorator on key A, abort lower pri) child1: selector1
child2: sequence1
selector1
(decorator on key B, abort lower pri) child1: wait task1
sequence1
child1: special BP task that waits a bit, sets key B, waits, sets key A (in that order), waits and finishes with success
child2: wait task2
tasks executed with bug: special task, wait task2
tasks executed without bug (observers reacting correctly) : special task, wait task1
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-31307 in the post.