Mmm, yes I've heard that as well. Aradia mentioned that non-humans are unable to keep their original forms here for whatever reason. I've been trying to come up with a suitable hypothesis for some time now, but it's rather difficult to do so. It could, perhaps be an effect of the atmosphere or maybe the work of some kind of Pokémon.

I wouldn't doubt that suppressing abilities could be done by a particularly gifted and powerful psychic type. All they would really have to do is trick one's mind into forgetting skills which it once learned. Like rewriting the brain, basically. It could be a sort of defensive mechanism to maintain the power balance of this world.