The AW really needs a way to manually calibrate the altimeter and/or calibrate from an onboard digital elevation map and GPS coordinates. You can’t rely on a barometric altimeter otherwise.
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Ah, thanks so much @ddmiller for the tip - configuring the Action Button makes starting the workout so much easier! That would have saved me so much fiddling and frustration, but of course will be beneficial going forward 
Hear hear, and in fact I have been thinking about the thread you started. The G-Shock 9400 Rangeman and many other watches have the facility to calibrate the altitude and barometric pressure, so it is very unlikely that this capability is not possible in the Apple Watch.
As I said in that thread, the Garmin watches I’ve owned all have multiple ways of calibration. Certainly Apple should be able to do it.
Currently my Ultra is about 30 ft low, which is acceptable. It’s usually within acceptable bounds, but about once a month it loses the plot and can go hundreds of feet off. When that happens, there’s nothing I can do about it.
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Any altimeter based on air pressure must be calibrated or it wouldn’t work at all. So the question is what Apple is doing if they don’t provide a mechanism for you to do it manually.
I’m guessing that they use your GPS coordinate in conjunction with a cloud-hosted weather database (to get the sea-level or ground-level barometric pressure). Which normally should work, but (like any cloud service) might fail if there are network or server-side problems.
This is, for example, how they calibrate the compass for the Earth’s magnetic deviation from true north.
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Just as an update, I’m finding that this gesture is working with the timer far more reliably in the last few watchOS 10 releases. (To tl;dr, this is the Settings / Accessibility / Quick Actions double-tap gesture support available on any watch running watchOS 10, not the double-tap gesture specifically for the Series 9 and Ultra 2.)