Take a look at the definition of The Gift on the glossary on page 8 of the corebook: "the ability to use almost any form of magic, with proper training. All magi have The Gift. The Gift makes people around a character uneasy." Social penalties are there from the very beginning, so the way I interpret it they just are part of it, something intrinsically tied to it. To see it as a lack of magical higiene that can be removed with the proper water and bathing salts seems odd to me. Moreover, it seems like a story seed regarding magi wanting to be socially accepted and unable to do so because of their power: what price would they pay? Just to take a bath from time to time there seems like putting the bar quite low.
Which is ok. I hadn't read about these baths, but I'm guessing there is no mechanical explanation on how they work, so let's make one: the only known way to get rid of The Gift's effects is to have The Gentle Gift (which reminds me of this discussion from a while ago, about how The Gentle Gift could be seen as a weaker Gift, which is what made me think that maybe by bathing there you would actually be weakening your Gift, and how that could work), which is a Virtue, that just lasts for a while there.
Powers can grant Virtues, and supernatural creatures have powers. I'd bet that there is one involved. When granted virtues are permanent, they drain the might score from the power's owner... but whey they are temporary, they just need to spend might points, as with any other power. This seems to fit with that bath so far.
The hypothesis suggest an experiment: go there with a magi with a very, very high Parma, and have him take a bath and see what happens. Also some InVi spells could come handy. And the realm affiliation of that possible creature would need to be checked, because it might well be a demon after all.
A good thing of this is that it doesn't even need to be true. If that bath is discovered, at least some magi are going to follow this line of thinking, and following them and their doings around the baths would probably give for a few stories regarding the baths.