When testing multiplayer in PIE (Play in Editor), I noticed that some of the API calls would error in ways that did not make sense. And these errors would not happen in single player testing.
I tracked this down to the use of the static singleton within
void UPlayFabClientAPI::Activate() { IPlayFab* pfSettings = &(IPlayFab::Get());
UE4 has a strange habit of having each editor-spawned client share a process. Due to the use of this singleton, each client was sharing the same PlayFab session ticket, among other things. Easy fix on our side, thanks to provided source code, but I figure I'd point this out.