The short flag stop at PHL suggests that a relief crew was called in and simply swapped out. Anything else would have required CBP to process the passengers at PHL and that's not likely to happen in <45 mins.

I do like that all three legacies are now doing a better job of getting close to destination even when the crew times out. Used to be that the flight would have terminated at LHR if there wasn't time to make it CLT.