Haha nice! The only thing that comes into my mind is that when you are recording an activity, ascent and descent values update a little slower then other measures, so it may happen that for a limited time it doesn't match your current status (eg: while climbing a small but steep ascent in a trail run, you see the ascent value stuck for a bunch of seconds and then it catches up the ascent a little later when you are already on plain ground). Maybe this is the same case, otherwise IDK
I have tried today do edit waypoint's altitude from where i start and now it seems correct, so the problem was that i saved a waypoint in the watch with a different altitude... it seems however strange... i don't know that every time i pass near a saved location with a known altitude, the watch calibrate itself even if automatic altitude calibration is set to off
