Z gets progressively deeper in 3D Engrave Finishing Pass

I’ve already posted on the FB group, but no solutions, so I figured I’d try here…

I have an issue where a 3D engrave starts cutting deeper as it progresses. My last test was 5" wide, 90-degree raster so it would cut cross-grain, and the thickness on the right (starting) side was a good 1-2mm thicker than the left (ending) side.

I just did a test using Offset Climb strategy (spiraling from the inside out) on a different model and it did the outer section first and then the inner and finally the center… but you could see the outer ring (first part to be carved) was a good 0.5-1mm thicker than the inner area.

My spoil board is surfaced, my Makita router is trammed as well as I can make it (surfacing left visible stripes, but nothing you could really “feel”), the 3D engrave otherwise looks beautiful… what could be going wrong here? The Z depth consistently got deeper across 2 different 3D models. Any help would be very appreciated!

I’ve had this problem twice, one time the lead screw started to slide down in the coupler, and the other time the anti-backlash nut was loose.