Z either losing calibration mid cut or too much backlash in lead screw?

Welcome to the forum. I haven’t had this particular issue, but I’ve seen it discussed multiple times on the forum and it almost always comes back to one of a few things. So far I haven’t seen that symptom due to backlash or calibration - they would be the least likely culprits.

Here’s a couple from the Related Posts suggestions:

Good luck and hope you get it cutting like normal again.