Ok ..some further information/possible clues -

The output voltage from the spindle drive controller doesn't vary ... regardless of what speed its set at. Its always around 96 volts, and I guess that means speed is frequency or current controlled.

When instructed to stop spindle ... the spindle slows but does not stop unless rpm's are low .... however whether its in hi or lo range also has an effect on what that actual rpm "trigger" point is. That appears to indicate whatever its doing is related to the Hall Effect switch (or something similar it is using for a reference/home/speed/direction) ??
The only way you can stop the spindle turning is to turn the spindle speed knob on the operator panel way down (near off but not quite there)

I have experimented with hi/lo gearing and its effects when commanding M19.
If its in hi range... the spindle will just keep hammering on the orientation arm bearing till it times out or you stop it using the spindle speed knob
If its in low range .... the spindle moves very slowly (looking for the orientation switch ?) and if its starting point was a long from the switch point .... it will overload the spindle motor before it gets to orientation position. If I manually turn the spindle so its "start" point is nearer the orientation position and do an M19 .... when it gets to its orientation point/position, the orientation arm/lock tries to engage but simultaneously, the spindle speeds up to "normal" speed and it overruns its orientation point ...

Looking at the overall picture it really looks like it might be the hall switch .... even though it appears to test ok (under DI DS). It maybe weak signal wise and/or perhaps there is "noise" on its signal line ... which might explain why its moving slowly when looking for its orientate position ??

Unless someone can shed some new light on the problem, I think I should get a new switch flown out and see what happens .... ??