Uneven Depth of Cut along x-axis

Shapeoko 3 xxl / HDZ

This issue just arose and I cant solve it for the life of me. I was cutting a pocket on a sign for a friend. Ive done many of these with no issue. As it pockets to the right side, its cuts more shallow. Especially towards the back right of the stock.
I made sure everything was square and level. All belts, pulleys, and v-wheels are appropriately tight. I ran small test cuts in between each adjustment so if it was fixed, I’d know the culprit. I made all mentioned adjustments to no avail. My test cuts still come out like this…

The top of the picture is towards the left of the x-axis and the bottom is towards the right, where the pocket gets shallow.

If this is repeatable, double-check your Z zero right after this happens. Is it off then ?
If it is, either you lost steps on the Z stepper motor (unlikely for this type of effect), or the Z axis pulley slipped (possible and a common issue, but you said yours is tight…did you do the marker test to be 100% sure?), or the endmill slipped in the collet (maybe the most likely root cause? is the collet dirty ?)

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My experience - order a new collet. If it starts to slip… hard to fix.