Barometr and altitude change on starting activity

I do know the current altitude so I have it set precisely, I don't need an extremely rough GPS 3D fix when not asked, GPS has never been good at altitude. So if it isn't a bug it's an overlook on Garmin's part.

Well, it's not. Most users don't start all their outdoor activities at the same spot and they often don't know it's altitude hence in most cases the GPS based altitude is the best available option. In most cases it's not the absolute altitude but the elevation gain or loss what matters during an activity and this doesn't depend on knowing the exact starting altitude. Also this can be later adjusted while reviewing the activity online (mentioned in my fist comment). And last but not least some models allow for manual altitude / air pressure calibration which overrides the GPS set one. So in my opinion the way Garmin handles this issue is excellent given the technological limitations.

I agree that you may not know your current altitude, but that is exactly what "Auto cal during activity" option is for. It is logical to expect that if I don't want that I keep it off and expect the watch not screwing up my set altitude. What that option is there for at all if GPS still tries to set your altitude when it's off?

Sorry to say but you apparently still not get it. The auto calibration periodically adjusts the altitude information to eliminate its changes based on air pressure change. In situation when you go for a 3 hour activity the air pressure might change significantly which would then result in very distorted information about the elevation gain/loss. The GPS position fix in the start of an activity is just setting a starting point but not adjusting it during an activity. So what you are looking for would be a setting which excludes the altitude information change from the initial GPS fix. Not sure this is possible, this would be a question to Garmin developers.

your only option in this case is to go into the activity settings and disable GPS during the activity, then with auto-cal also disabled you have full control over your altitude settings.  I would agree with savinoff that having another option to disable altitude adjustment by GPS during an activity would be a worthwhile menu option, since for activities like hiking if you have a good map and knowledge of altitude points along the route you can do better with the barometric altimeter than using GPS elevation estimates.  I have GPS turned off during hikes, and can live with the fact that I can't track my hike with GPS, but other people may not find that compromise to be worth it.

I’ve noticed some very strange fluctuations since the latest update (was fine on 10 RC but not 10.10). Will keep checking on next few activities but yesterday for first time it showed 100 metres extra climb on run and then today it went up to 100m at home from usual 52. This was an issue when I first got the watch (at release) but it’s been spot on for months now.

That's exactly my thought - the option does say "At Start" so the behavior I'm getting would make total sense IF I had it ON. I have it turned off though yet it still "calibrates" at start to some pretty random value.