GetInstanceTransitionTimeElapsed doesn't use TransitionIndex.
So, if Animation BP run few few transitions at the same time, GetInstanceTransitionTimeElapsed will not return right value. It will ignores the location of the Transition and returns the latest value transition's value. This is misleading.
As far as the past update history is seen, it is probably due to omission from the target of batch replacement.
Use GetInstanceTransitionTimeElapse in a transition rule
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-136117 in the post.