I think I had a slight change in mines behavior since iOS 18 went GA. I have been running just as much, if not more, and had far, far fewer (but still some) misreads. I can't be sure if it's iOS 18, WatchOS 11, or something else entirely. It used to happen on 90% of runs, now it seems like 10%.
Safe to say the problem is not solved, but for whatever reason is cropping up less for me.
It does seem more likely on longer/harder runs, so sweat in the sensors could be a factor. I don't sweat a ton, like never dripping sweat or soaking my watch band even, but it has been cooling off in my area making the sweating even less.
@24yhtomik are you sure this inflates calories? As a test, I went to a prior day with too many stairs, and deleted those from health, and saw that calories burned did not change. I did a long run that day of 20 miles, and had around 3100 cals burned. The run accounts for 2,344 cals. If I had actually climbed up the recorded 200 flights of stairs, my calorie count would likely be 2-3x as much. That's an insane amount of stairs. Like you would need to be fit as ****, and spend hours climbing. It doesn't seem to me that the stairs impacts the cals. Calories is probably some function of HR * time. I haven't seen anything the flights climbed would impact. More of an annoyance.