A few weeks ago I hopped on a plane and my altimeter on my Watch has not been the same. Either it wouldn’t show a value (just three dashes) or it shows a wildly incorrect one. (off by 2,500-5,000 feet)
A live view of my elevation while hiking was one of the factors for upgrading my watch so this was a bit frustrating. I tried a lot of things on my own, and finally ended up contacting Apple support for the first time in … ever? Even unpairing my watch didn’t do the trick.
Last week I let my watch sleep overnight turned off. In the morning I decided to try something different. I turned my iPhone off, then I turned my Apple Watch on.
That was about a week ago. It’s been correct ever since, even through two additional flights.
Hope this helps someone out there.