Well, just in case you ever decide to give it a shot, here's a save reproducing it outside of challenge mode, in a one-on-one duel. Activating
Mirror Universe with the
Lich out sets my opponent's life to 5 this time. And this time there was no life gaining or damage happening -- all that happened was me casting
Lich (with a
Dark Ritual), then casting a
Mirror Universe, then using it.
It's weird, since I think both
Lich and
Mirror Universe were in the original game, and this was a well-known combo even back then; so I guess this is a bug that was present in its entirety even in the original version.