Nope wasn't the X-mirror

I am so confused. Every time I think I've figured out what is causing the problem, it does something I was not expecting. The X-mirror thing was not accurate at all. The best guess I can come up with now is that its something about the way that the bone weights overlap. But its difficult to figure out exactly what the "rule," is, so to speak. At one point I thought that it was "Only adjacent bones can have common weight." But that's not what causes it alone; I did a test model of a segmented rectangular prism where weights from a grandchild bone overlapped with weights from the grandparent. It imported fine. I really hope that soon everything will become clear.