Yes, it does. I also failed to mention that those two mares are not the only horses I know of that reacted. Those are just my personal experience and I have heard of even more negative changes, usually mostly behavioral and horses being more prone to developing insulin resistance, while on a soy based feed or supplement.
I see a lot of parallels between critters and human related to diet and health. I have learned so much from the animals I have had the honor to know.