I've seen loads of threads complaining about inconsistencies in the floor counting on Galaxy watches; all of them seem to be unsolved (asides from Samsung's poor effort at just telling its users to reset, recallibrate and refresh everything (oldest story in the book), standard I.T. bod "turn it off and on" rubbish )!!
Haven't had time to look through all the threads to see if someone has spotted a fix, so this might have already been talked about...but, I think I may have cracked it!
If you go into the "Alti-Barometer" app through the watch, when it's on the "Altimeter" screen you'll see a last refreshed status at the bottom. If that hasn't been refreshed for a while, the watch will either assume you're much lower than you currently are (which would give wildly high floor count reads) or much higher than you currently are (giving no reads at all for climbing stairs or going up a hill!). Scroll over to the 3rd page in the app (past the "Barometer" page) and you should see a list of settings; click on "CHANGE" and make sure you turn on "Auto refresh". Et voila!
This has worked for me...I hope it brings solice to all you other Galaxy Watch users