[Alan Anderson]

Check the power LEDs on your Digital Sidecar. Make sure they are all brightly lit, especially the one labeled BAT.

The "full forward breaks, full reverse works" description is consistent with a missing 12 volt power supply on the Digital Sidecar. The sidecar can steal a teeny bit of power from the PWM signals coming from the NI 9403 module and make things sort of work, but the available power is reduced when the signal goes from neutral to forward.

Even if it doesn't work in Teleop, it might work in Autonomous if you haven't initialized other motors or if there are other differences in signal loads.