Bug? Output randomly changes

Situation : Cue running in playback, only 1 cuelist active. One or more of one fixture type (Ayrton Mistral extended) is in use, with a mixed hue/sat colour. The colour wheel is at 0% (open)
Select any active Mistral, alter the intensity (not touching any other kind or parameter)
Merge the new data into the cue.
At the point I hit 'Enter' at the end of the merge command, the unit I have edited (and sometimes ALL live Mistrals) snaps its colour wheel to red. The DMX output window shows the change - the units' 18th parameter value moves from 32 (open) to 28 (red) and the output screen shows the colour wheel value change from 0% to -338%
If I skip back 1 cue and then skip forward in the cuelist the cue replays correctly, with the new intensity and the existing tracking value for the colour wheel.
This happens whether the mixed colour in the unit is from a pallette or hard values, and whether the colour is in the cue or tracking.
I've used intensity as an example here to show that it happens when the edit doesn't contain any colour data, but it happens on any edit. During programming this even got to the point where if I changed the cue time by clicking the cuelist cell, the colour still went red.
Desk is 3.17.0 from a full install. Show file is a 'startup' file created in 3.16.0 and merged into a blank 3.17.0 show. The cuelist was programmed in this 3.17.0 show. 

This has meant that I cannot make any live changes to these units during rehearsals or shows, it's driving me mad!
What is it doing??

Videos :
vimeo.com/657596262
vimeo.com/657599184