This is an issue that was reported in a previous thread about a year ago, but that thread has now been locked with a recent comment from Garmin-Sierra that "we believe this has been fixed for a couple of months". (See https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-955-series/354218/backcountry-skiing---problems-with-automatic-mode-tracking-in-ver-17-26/1857797).Unfortunately I think that was optimistic. Here's a recording of mine from this morning:
As you can see, the device recorded this as one long "ascent", even though there were obviously multiple ascents and descents, with speeds of up to 55 m.p.h. on the descent. I'd like to see someone go uphill at that speed! For activity details, see https://connect.garmin.com/modern/activity/17679748186.
This is just a guess, but I suspect that Garmin's auto-detect algorithm is being fooled by relatively fast transitions. In this case my uphill-to-downhill transitions were about 1 minute, and downhill-to-uphill about 2 minutes. (That's not very fast! An elite skimo racer would be two or three times faster). However, you'd think that their algorithm would recognize that it can't possibly be an "ascent" when someone's elevation is dropping by hundreds of feet every minute and their horizontal speed is 55 m.p.h.
