Having the exact same problem here! The only work around I’ve found is increasing temporal samples while keeping spatial samples at 1. You’ll have to increase the interations and subsamples of your Groom’s physics sim by a factor of the number of temporal samples you’ve added. If you encounter any bumpiness between frames try increasing the compression error threshold scale in your animation asset, I also turned off frame stripping for good measure.
Temporal AA cleans up your image a little bit more than no AA at all but it also creates a few new problems and adds way more time to your render, so I guess try to find a balance you’re happy with.
Ultimately, turning on spatial AA shouldn’t break Groom Sims but at least there’s something of a work around.