I suspect this may be a bug.
I have a simple build with MATEK F405 WING and BEITIAN BN 880. No additional sensors. AC3.6.7
I noticed that as soon as copter starts moving it keeps gaining altitude until it reaches the destination or a WP, then it descends back down to the ALT of the destination / WP.
Example in attached log:
my base elevation is 42 m. Guided (and mission) alts are relative - 100m .
POS,305994941,42.52309,-71.1865475,179.11000061035156,100.79630279541016,101.32630157470703 ... --requesting new position at 100m GUID,306027357,1,425226176,-711864576,10000,0,0,0 ... --starts moving: POS,306094937,42.52309,-71.1865475,179.08999633789062,100.77618408203125,101.30618286132812 ... --now copter transitions to the requested destination - already 9m above target: POS,317794932,42.5226417,-71.1864785,187.6599884033203,109.34822845458984,109.87822723388672 ... --reached the dest at 20m above requested alt POS,322394929,42.5226202,-71.1864528,197.51998901367188,119.20286560058594,119.73286437988281 .. --starts descent and reaches the original atl: POS,336394951,42.5226178,-71.1864569,179.97999572753906,101.66571044921875,102.19570922851562
when i run similar experiment with pixhawk cube and here2 everything is fine - no altitude gain