Z axis moves below zero then returns and cuts normally

Hi everyone

I just upgraded my Shapeoko2 Z axis to the X carve Z axis (incl reversing the motor polarity) It was all working properly before i upgraded but now i have a strange issue where i zero the axis, I then start the job and the Z axis drops below zero, moves on the Y and X to the first cut, rises back up to above zero and starts the cut. From this point the axis moves in the correct orientation and steps down while cutting like it should.

It has got me baffled… It must be something simple…

Any ideas? (I am in New Zealand so prob opposite timezone but i will reply!)

Is it possible that reversing the Z axis movement has fouled up your G-code setup for moving to the work piece? Just a WAG (wild ■■■ guess).

Maybe you accidentally reset your safety height to a negative number? Are you using Easel? I doubt that would even be possible in Easel, but may be possible in a more complex program with less restrictions.

Are you by any chance using Fusion 360 and seting your work zero at the top of the spoilboard instead of the top of the work piece?

Thanks for all the advice!
I am using hsm express, I tried cutting a job I had successfully cut prior to the upgrade and the same thing occurred. But I will redo the cam and try again.
Worst case I will redo the firmware on the tiny g and see if that helps