Altimeter jump from 913 meters to 20000 meters

You gotta stop wearing those rocket shoes!

Recalibrate outside away from stuff, see if that helps.

This sounds a lot like the symptoms of a broken barometer. If it's anything like the various other broken barometers Garmin watches are prone to then there's nothing to do but phone for a replacement.

either that- or it's the same insidious problem that the Fenix 3 series has always had. Recalibrate. but- it's still an intermittent bug that Garmin knows about and hasn't been able to fix. or- you could have a broken altimeter (pressure sensor)

My altimeter has been real wonky since v6 was installed. Previously, it was generally pretty accurate... now, it's consistently reading 300+ feet higher. Plus, my stair count has been way off. I'm hitting my goal like 3 hours after waking up, so I know the altimeter is fluctating a lot, causing false readings on that.

You gotta stop wearing those rocket shoes!

Also stop jumping off upside-down mountains!

I too had a problem with altimeter calibration this morning on my first run with v6.75b. It was showing more or less a correct altitude (around 1050 ft.) before. But I noticed altitude was way off (~ 200 ft) possibly due to auto-calibration after the run. I turned off Auto-cal and calibrated the barometer manually with altitude and sea level pressure (using airport METAR data). Will see how it holds up.

Got my FR935 replaced with exactly the same symptoms (see the images). Definitely, the faulty barometer.