Only the last touch event's location in the frame is used to send all events for a given finger index. A finder index may be reassigned after a release by another press in the same frame from a very different screen location.
Touch locations need to be stored and passed on with the proper event in PlayerInput.cpp.
See the UDN report
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-21219 in the post.