This one appears to relate to different airfield elevations between simulations.
I was in XP11.10b7; my colleague was in P3Dv2. We were both parked at Bristol. He was at 625ft AMSL, I was at 602ft AMSL on account of having "runway follows contours" turned on.
Whilst his JoinFS instance was being sent my "real" altitude as confirmed through the "Aircraft" dialog, in his simulator he saw me rise out of the ground and then keep rising to around 800ft before I disappeared below ground again and the cycle repeated. All non-JoinFS software he had attached to his simulation reported the same (as you'd expect it to). I was completely stationary - no engines running and parking brake set.
My guess is that I was getting a vertical speed from somewhere through being moved at his end from below the ground to being level with it, and that vertical speed was retained and used to modify my altitude in his sim until such time as it got a new position / altitude update. The issue went away when I taxiied to a part of the runway at the same elevation as his scenery - and of course when I turned "runway follows contours" off - both of these scenarios resulted in us being at the same altitude AMSL so there was no correction going on.
We probably didn't do enough testing to work out the effect that the "Elevation Correction" option had on things, but it changed nothing when I flipped the state from "off" to "on".
That's it for tonight!
