Animagus transfiguration - permanent or not?

In HPMOR canon transfiguration is not permanent. Animagus transformation is also a kind of transfiguration, albeit not a free transfiguration.

HPMOR also postulates that long-term transfiguration can only be done by sustaining it constantly with a supply of magic. Thus, if transfiguration is not sustained, sooner or later it will be reverted to the original state of the object.

And we also know that when a person dies, their magic permanently leaves the world, implying that whatever transfiguration they sustained will revert some time after their death.

Now the question is - why we never heard anything about Rita Skeeter's corpse suddenly appearing in Mary's room some time after she was killed?