[Alan Anderson]

This is probably not your problem, but it's easy to check: is the Digital Sidecar getting good 12V power? Look to see that the 12V LED is lit. Measure the voltage on the white WAGO power connector.

(The reason you might see this behavior if there's a power problem is that "sneakage" from the PWM signals is barely enough to keep the Digital Sidecar sort of working...until you call for reverse, and the pulse width drops.)