15mm is a lot of steps out.
Yes, swapping components is easiest to test.
Swap wires first, drivers second, motors themselves last and test (when the motors are off) for how much force is needed on the coupling to see if one axis is binding somewhere.
Actually, check for loose wires first.
But, if the size of layer 2 and 3 are ok, then it might be interference from the z that is causing the issue.....that's less likely than just loosing or gaining steps along the cut line though.
Good luck
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