YL Motor Only Moving in one Direction

Just finishing up the build after a couple of shipping mishaps. I’ve shimmed up the proximity sensors and got them sorted, and modded the non-mirror L-bracket that some of had for the drag chain transition.

When I first went to initialize the machine - the two Y motors were running in reverse of each other. The Z axis homed properly, but as movement started in the Y-direction the machine starts to fight itself pretty violently. I pulled the YL connection, and can Initialize with just one Y stepper (YR) and the machine runs (and jogs) as I would expect.

I figured that the left stepper motor was wired up wrong, so I flipped the Green / Black wire pairing to reverse the stepper direction. When initializing with both Y motors, the Initialization will run properly. But - when I try to jog away from the back of the table, the motors are running in opposite direction to each other (YR in the desired direction, YL towards the back of the table).

Plugging just the YL motor into either of the boards Y motor connections results in the same initialization result - the gantry moving towards the back of the table.

I switched the White/Red pairing on the motor out of curiosity, which results in the same problem as the start - the YL motor only running towards the front of the machine regardless of the input.

Any thoughts?