I know, I know, the first gen Instinct/Solar doesn't come with the Backcountry Ski activity. This is a travesty. There's no reason Garmin can't release this for the OG Instinct. Leaves a bad taste in my mouth for sure.
However, I got the file over the summer and dropped it into the appropriate folder on my watch. Did a test, worked fine. Did a tour in November, worked fine. Did the update to 18.54 over the holidays. Went out to tour today and it was acting wonky. I'd go for a bit and all of the sudden, distance and vertical would get hung up. Time was still running, HR still working, but distance and vertical were stuck. Once, I saw it adjust, but for the most part, just stuck. Very similar to what's being described here: https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/fenix-6-series/255346/strange-elevation-profile---ski-touring-activity
Just tried taking the file off the watch and reloading. We'll see if that works next time I get out. Really, Garmin just needs to make the activity available on the gen 1 Instinct and support it in future firmware updates.