It never, or seldom happened till October-ish. It was, in any case, after New England sailing season when I relied on it daily and never noticed this. Since then it’s perhaps every other week, on BOTH my iPad and my iPhone. It’s not related to restarting my device, and seemingly not related to Apple software updates which I always do immediately on release. I’ll keep paying attention. I have the app as a widget on both devices, so I usually notice quickly when the widget goes blank. Fortunately logging back in is easy, but losing US settings - everything going back to metric - is annoying.

I use Apple to log in as a Premium user, but regularly get logged out, and lose many of my settings. I.e. it switches me to Celsius and meters, yet remembers my favorite locations. This happens on both my iPad and iPhone. At first I thought it was just when there was a system upgrade by Apple, but now I realize it,s more frequent and seemingly random.