I can't seem to find the datasheet for the TL421... Digikey, Mouser, and Octopart don't turn up anything; and google seems to be under the impression that it's a front-loader for a Kubota tractor.
As for the saw-tooth, I'd need to see the datasheet, as well as the schematic of the circuit before I could hazard much of a guess as to what is going on. Op-amp circuits can oscillate; but my understanding of it is that usually it takes some external components to make a saw-tooth - by itself, you usually get a sine wave if it breaks into oscillation.
It's less critical than one would think; because of the feedback from the encoders (if it drifts out of position by as little as one encoder count, the feedback loop will command a rotation in the opposite direction until it's back where it's supposed to be). Also, you can include a 10-turn trimmer pot to adjust the "zero point" of the reference voltage to match the zero-RPM output voltage of the SAMD21. However, better is, well, better... so...I ask all this because I feel a very good reference voltage will be critical to the motor drivers functioning at their best.