My apologies - I've read the OP again, and I did slip up and write feedrate override, rather than spindle override.
There are spindle override buttons on the bottom of page 2 I believe (can't open the screenset from here). I orignially identified the issue by switching to the 1024 screenset and using the buttons there.
I have the same issue when entering the M3 S400 command in the 1024 screeenset, which is not running anythign else in the background. Yes, my code does organise some motion, but even without the code doing anything, or using a screenset that has minimal code it still doesn't work. I will have a go at turning off the macropump in the general config, and then starting and stopping the spindle and changing speed from the 1024 screenset, just to check that there is no strange interaction there.
I'll try opening in Mach Screen and looing through for duplicate DROs, is there an easy way to do this ? I'm not quite sure what you mean, as there are two versions of one user DRO on a page, one as a readout and one as a slider.
Tim