Ok
Where and how do you touch off the part ie. at the bottom or the top and are you doing the same in fusion360 you generally touch off at the top off the part and that should be Z0.

Also when you jog the Z axis down dose the DRO go more -ve or +ve ie. generally the Z should go -ve when going down.

I have never used grbl.

Also in the code you are using a ball nose cutter for a pocket you should be using an endmill . This is nothing to do with your problem though