I recently resolved a similar problem with my CNC. I have two motors on my Z axis, and one would quit turning one direction, so that even when I changed directions, one motor would obey, and the other would not. The result was an instantly racked gantry.
The problem is slightly different, in that the motor in question is NOT turning in one direction. In your case its turning in the WRONG direction. Be sure that is the case. In my case I had to disengage the motor from its rack gear to clearly see the direction of the pinon gear.
In my case a motor winding was not getting voltage, which ended up being a bad connection at the Gecko driver for that motor. When I pushed on the connector I got the motor to work correctly, but only intemittently. I replaced the Gecko and installed new connectors and its been working fine.
But I have to add that I probably BLEW the Gecko by prodding around at the connector when I was troubleshooting. It sparked once, and then quit working altogether. I guess that made the repair easier, but more expensive.
If you are using Geckos, try swapping out both the Gecko and the connectors. I now keep a spare Gecko driver so I can do this.
Good luck, and don't get too frustrated. It'll fix.
Wayne from White Salmon