From reading the forum, lost Z steps are one of four things:
- Mechanical issue
- Feedrate too fast
- Current setting to the drivers too low
- Wiring issue
Mechanical issues are usually some sort of binding in the lead screw. Less common is some sort of obstruction like something rubbing against the router as it moves up and down.
X and Y axes use belts on the MPCNC, but the Z axis is a lead screw. Led screws cannot be driven as fast as the belts. The forum saw a number of issues with lost Z steps when Fusion 360 eliminated Rapids, Z movements ended up with too high a feedrate for Z in some situations. This can be mitigated by limiting the feedrate in the firmware (M203 g-code followed by a M500 to save the setting). Post the g-code for a project that is failing, and someone can examine it for feedrate issues.
Raising the current on the steppers have solved lost steps for some users. Since the same current settings work for other MPCNC machines, I suspect there is a minor mechanical issue that raising the current hides, but this is only a guess.
Note that for the first three issues, it is common for the lost steps to be in the up direction but not necessarily in the down direction.