One thing you could try which has worked for me in the past is look at the Macro program for a tool setter movement or probe movement etc and see if there are any M codes there that are unusual.
Obviously a tool set movement or probe require the spindle to be still, some machine builders just make their own M code up for "movement without spindle rpm"
I had a machine once that I just could not get to do a ballbar without spindle running when I had the idea to look at the tool setter macro and it ended up being M300 and the machine next to it with the same control from the same builder was M194