If it isn’t too much work, try removing the belts one at a time testing to see if that fixes the problem and if the motors are spinning correctly. If one motor isn’t working it will take a lot more work for the other motor to overcome it. Also if there is any sort of a short on a stepper it might cause it to lock up making the other one work a lot more.
If that isn’t it, then I have no clue. Maybe the quick disconnect you added is messing with things.