Why did my soft tofu deflate?
I don't know for sure, but I have a theory.  Tofu contains a lot of water or soy whey.  Soft tofu contains even more than firm varieties.  This water is trapped in the cell structure of the soy proteins when they coagulated and were pressed together
Freezing causes water, which has now, of course become a solid, to expand. When you simmered you tofu in warm water, you very quickly thawed the tofu, rupturing the cell structure.
We know from examining the thawing of other frozen products, like chicken breast for example, that quick thawing leads to higher moisture loss than slow thawing in the refrigerator.
So, I think, in your case, you rapidly destroyed the structure that typically keeps the water in place.  The water left the tofu, and tofu flattened.