If it is OS related then you should worry cause if iOS users can no longer use SkyDemon then you will loose a large user base. I use SkyDemon for some years now and never had issues up until now. I use both the internal GPS and an external GPS (Garmin GLO). I also experience the problem that sometimes the whole flight log does not show a correct altitude (within acceptable variation due to GPS altitude detection) Not just a dropout but the whole flight. It does detect the ascending and descending phases of the flight correctly. When I look at the flight profile afterwards it does show the correct positions and corresponding changes in altitude but the altitude values are way off. For example 3ft up to 700ft is recorded as -13ft up to 13ft. 

I can't remember when the issue started to manifest itself but as another member already mentioned it started to appear after iOS 12. I'm not sure but I do know that I had no issues prior to iOS 12. 

I'm using an iPhone 8+ and older iPhone 6. Sometimes paired with a Garmin GLO and the altitude issue also occurred when using the external GPS. So it is not related to the GPS hardware. I really do think this is a software issue and probably is related to a change in iOS.