3d print lean in Y-axis
I would assume it is either the stepper driver or the stepper motor. Try switching the motor wires for x and y axis and see if the problem stays with the motor or the driver. If its not physically getting hung up then this is likely. I had the same problem with my y axis and after switching the motor it was gone. It would only skip steps in one direction and that seems to be exactly whats going on. Most likely it was caused by bad windings on 1 of the 2 coils inside the stepper motor.