It seems to me, if you grant a copyright contingent on some royalty, if the royalty is not forthcoming, then the copyright must revert to the author ... is that not true?

And if that is not happening, then it means the author never held the copyright, but rather had a royalty that was based on some other mechanism. If the author did not have copyright, it means they must have been doing their work as a work for hire.

This does not change the fact that Disney is a bunch of scum bags.