That is how I wired it for split. Only difference is my grounds are grounded to a grounding block from the MPs AC ins and outs and then grounded to the frame.
Wired that way, then configured for split, the slave overloads (overload light comes on and it turns off
The only explanation I can think of is that it's not actually wired correctly. Are you able to take pictures such that we can trace the wiring? And perhaps a screenshot of the configuration that shows the split-phase config?