@OwenD Unfortunately that's not the issue, at least with those of using using slightly odder part cooling.
More information in general for what's happening specifically for me;
When I call a fan name in M106, the slider is in DWC while not printing, but doesn't actually do anything. When printing, the part cooling fan works as it should, but the slider sits at zero unless I touch it (and I do then have control), until the gcode changes the fan setting (at which point the slider stays where I put it and the fan changes to the speed the gcode called for). In this instance, the option for Tool Fan and the same fan as the name I've set in M106 are available.
If I remove the fan name from M106, it is not available while not printing at all (and neither is Tool Fan), but it otherwise functions the same during a print (Tool Fan becomes available).
If I run M106 P1 S0.5 while not in a print, it does work.
After updating to RC6, the functioning slider is back while not printing, but it's displaying the pre- X value set in M106. In my case, X0.3 and setting the fan slider at 50% will cause it to snap back to 15. Using the + and - buttons is sort of fruitless unless you click really fast.