Sorry, forgot about this. It happened me a couple of times, first time I took off from hill top approx 500-700 MSL and flew into the valley below, when I landed on the hill again instead of reading 0 it was 330ft.
Next time I took off from a large strand, my usual flying spot, and the altitude fluctuated wildly throughout the flight, I didnt have a second altimeter to check against but by eye the actual altitude didn't look accurate against the watch.
I had been using the app prior to the update and never had any issues, except with zeroing the altitude before takeoff, would only work sometimes, at the strand, at sea level, it always read about 200ft, I would just subtract 200 from the reading to get the correct altitude in flight. But when I landed the take off altitude and landing altitude were always the same.