Random Z-Axis plunge

[JAZZCNC]

Did it do it every time you ran the same code at the same point in G-code or occsionally and randomly with the same code.? If the later then I'd be surprised if it's missing steps.
Esp has it's exactly 10mm, missed steps from overtuning tend to be random amounts resulting in random offset distances.?

Seen has it's a constant figure It could also be backlash.? Backlash would give exact offset distances.!
Check all mechanical connections on the ballscrew, you should if built correctly have a fixed end where the ballscrew comes thru the thrust bearings and is fasten with a lock nut to stop end float. If this nut works lose(which does happen) it will have the same effect has backlash and give a constant movement amount when changing direction.!

EDit: The movement amount can be tiny but it accumlates with lots of moves resulting in an exact amount at a set point in the job so check carefully because even a tiny amount is bad news.!!

It's the constant 10mm amount that concerns me, missed steps thru overtune don't often give same error amounts.?