If the system uses switches for steer lockout, and if its a zapi dc series wound motor controller, then i would expect one of the motors to stop in one lock.

If its not dropping out and the switches and wiring all check out ok, not sure what else to check. I would take a guess and assume the controller should see a feed or have a feed removed by the steer switches which in turn would remove the feed from the contactors (if its a series wound traction motor).

Possible faulty controller??