I have the same type of CPAP, I can't go under 15% too, the board seems to have a minimum voltage startup. @dhusolo
Maybe you can configure in RRF 0% for the slicer matching 15% for the FAN to keep easier slicer setting to avoid false 0% with 10% set in the slicer. @dc42 ?
If someone found the CPAP board schematics I could investigate deeply (I'm electronic engineer)