Is it possible that something, perhaps setup or maintenance related, is displacing the z-rod nuts? I.e., is something "unloading" the x axis from the z-rod nuts?

People have mentioned something like this happening when they've homed on Z with the HE still oozing. As the HE cools, the plastic/HE/bed become somewhat bonded to each other. Then, if a +Z move is made before things have heated back up, the z-rod nuts can back out of their capture holes, since the HE is somewhat stuck to the bed.

That's one way, I imagine there are others.

0.02USD...