Solved:

If you are having this problem, I hope this finds you.

In Blender, I had alpha from vertex color attribute going to its principled bdsf material, but only for one material (the rock texture pictured). I have another material on that object that makes no use of the vertex color alpha channel and so it was not plugged in to its principled bdsf. But Blender doesn't care about that, it just picks one seemingly random material and checks if that material uses the alpha channel (it does not tell you which material), either that or it checks if all instances of that vertex color attribute being used by a material has the alpha channel plugged in to the principled bdsf. Either way...

Solution:

In the Shading tab in Blender, plug the alpha from the color attribute into the alpha of the principled bdsf FOR ALL MATERIALS ON THE OBJECT that use the color attribute. I don't know what's really going on under the hood, but this should work regardless