SPI communication with DRV8908-Q1 motor driver
After a couple days of diving through the manual, the issue turned out to be with my testing method. According to the manual open load detection (OLD) will trigger if the current drops below a specific level (after a specified amount of time, 4ms max), turning off the chip entirely until re-enabled through the correct register. By default this OLD triggers at currents below (nominally) 9mA, but it could be as high as 18mA. (Figure is from page 17).

Turns out my 1k dummy load was creating a current of 12mA, right in the grey area, and for my specific chip, this was too low so the chip shut down. The solution was simple, either disable OLD, or use a different dummy load (in my case I used a 100R load with 3W dissipation, stuff gets toasty).
So once again, fairly simple solution, RTFM.