Update: I removed both y-axis steppers. Both had the red bushings installed, but the stepper on the left (side with too much play) spins almost freely, and the stepper on the right offers much more resistance when I turn the shaft. I don’t know what a stepper is supposed to feel like, but I do understand single-variable hypothesis testing. I have a feeling something broke inside the motor. I ordered a replacement to arrive tomorrow. I’ll update then.
I can see in the shape of the bad cuts how too much play in the y-axis would cause this, and a failing stepper motor would be the same effect as a missing bushing.