Hello,
After a long process of elimination I have found that if you have a damped path for position then the position parameters release from an IPCB fader when it reaches 100%.
Totally reproducible in a new show file with a single fixture stored with position, damped path and fader set to IPCB. When pushed to 100% the position values release.
All works fine in v3.19.1 and v3.21.0.
Since I always have a damped path set in my default timings I didn't realise the cause until poking around in Hog PC this evening.
Cheers
Colin