When I was home at Christmas (at sea level) my brothers and I rode outside once, and my NP was 235 watts, which is pretty much bang on my FTP (we tend to have at each other;-)
Fast forward 2 weeks and I'm home (1300m above sea level) do an FTP test on Zwift, and it tells me 210w. Heart rates were pretty much the same (165bpm-ish).
Does this smell right?
Larry