It was previously reported. I think the female form of Hippopotas and Hippowdon was not handled (it was actually form 1, and at that point I think it was programmed to only expect form 0).
The question mark appears whenever the mon in question has form ID larger than expected form ID.
Try updating PKHeX and see if that error is still there. (I'm not sure which version fixed it, if I'm being honest)
Anyway your Hippo is correct, so it's fine.