Up to 5.03 it was possible to directly write to directly write to sim/cockpit2/engine/actuators/prop_ratio[0] (range from 0 to 1) to set the collective from the outside (e.g. by a plugin). Since 5.10 this does not work anymore, as the joystick input on the collective axis overwrites ALWAYS sim/flightmodel/engine/POINT_pitch_deg_use[0] (which is linearly coupled to sim/cockpit2/engine/actuators/prop_ratio[0]) regardless if collective axis is used or not (detectable by reading joy_mapped_axis_avail[x]).

Less a bug than a missing feature. Implemented from V5.12.