I’m not an expert by any means, so take anything I say on this subject with a grain of salt.
The majority of soy is genetically modified, which makes it a ridiculous allergy food. I wouldn’t be surprised if your mare was allergic to it. I’d think if you can find a good food that does not contain soy and wean her onto that, you could watch her to see if the symptoms disappear. If they continue then it wouldn’t have been the soy.
I pretty much came in here because the title intrigued me, as I am deathly allergic to even non-GMO soy (which makes it really hard to eat, because practically everything on the market contains soy now).
Equine color genetics enthusiast!