Problems with DRV103 PWM
I believe I found the problem - one of the example circuits has a pullup resistor between OUT and V+ when working with a transistor. I added a 4.09k and now it seems to be working.

EDIT: The output was still not quite right, so I added a second 4.09k pulldown resistor between the PWM line and ground. This completely fixed it, giving a perfect square wave.