hi,
interesting.
you are right. in that case, it didn't calculate the flights correctly.
i have had similar issues with my Watch as well (Series 8; aluminium), but in a slightly different way.
in the days before i bought my Watch, my iPhone would calculate a stretch of mountain road i would sometimes run up to be about 22 flights. the iPhone (carried in a side pocket) would come up with this number very regularly.
after i got my Watch, it took about 4 or 5 days for the Watch to calculate a consistent number of flights. the number of flights the Watch consistently calculated was about 14 flights, however. according to netbased and paper maps however, the actual elevation change was closer to the elevation calculated by the iPhone.
i thought two different devices therefore two different sensors, so i was going to just accept it.
but, the Watch also gives actual meters of elevation change as well as the equivalent in flights. and the number of meters of elevation change it reported was similar to the meter elevation change being reported by the iPhone.
apple tells us to calibrate the iPhone and the Watch (through the setting on the iPhone).
i flipped the setting to off and then back to on.
after about 2 weeks the Watch then began to calculate flights correctly (or, at least, closer to the data being reported by the iPhone.
i see you are using the Health app to view the flight data. for me there was also the problem that the data being reported in the iPhone Health app did not match the flights shown on the Watch. but that problem also got resolved.
in the end, it took several weeks for my data to be consistent. i haven't heard of anyone reporting that it takes so long to report more accurately, but in my case it did take that long. even to the point that the Watch reports the Move (calorie) number more aggressively for running that stretch.
i now can take only my Watch with me on my walks and runs, without carrying the iPhone.