With you using an external controller then things may be different.
From my understanding the DROs in Mach are updated in a similar manner as the PP, when using an external controller. What I mean by that is, when using the PP it is the pulses output from the driver that updates the DROs in Mach. When using an external controller the same happens, it is the controller that will update the DROs in Mach.
The difference however may be that the external controller is updating the DROs but not actually using the pulse output to get that data. You would really need to ask CNCDrives to look into it unless you have a scope and can test for yourself. If you do have a scope then it will be easy to see if the pulses are being output from the UC100 when the motors are not, but should be, moving.
Hood