I don’t understand neither because I had my phone on my hip when I walked upstairs at work between noon-12:20pm and it calculated 6 flights. I had left my watch on the charger. The phone calculates it fine.
Been having similar issues the last few days. The watch either hasn’t detected any elevation gain or like yesterday, it gave me 26 flights climbed on a 6 mile run, which I’ve never seen happen before.