Found a workaround based on another post. PWM channels 5-8 do not work for auxiliary motors and Quadplane is using auxiliary motors for the quad lift motors. By reassigning channels 5-8 to 9-12 via the mission planner, the erratic PWMs are now gone.
One downside of this approach is that on the status page in MP planner, channels 9-12 are not updated and indicate 0.
Would be nice if the channel 5-8 problem could be fixed in Navio2.