Phew! Glad to hear it self resolved!
Yeah, that matches my suspicion.
What I think might have happened there is the app had updated in the background at some point, which would have led to it showing the subscription view. You didn’t see it at that time, because it was in the background. But iOS took a screenshot of the app, as it always periodically does, to have a placeholder to show on screen when apps are resuming from suspend or other various situations.
Sometimes when you open an app you’ll see some stale / outdated view, which doesn’t do anything when you try to interact with it, then it soon after (hopefully) disappears. That’s iOS showing that placeholder screenshot while the app is starting up or coming out of suspend or some such. Looks real, but isn’t.
So when you say nothing happened when you tapped the button… I suspect that’s what you were seeing. The UI should have at least changed in some way, to give you some feedback.
Anyway, what the app would actually have been doing while iOS was showing you that fake screen for too long was the migration. It would have already been busy doing the big nasty migration. You should’ve been seeing the migration view at that point, but iOS had different ideas.
So when you swiped it closed you’d actually stopped the migration part way through. Which… luckily is harmless! SQLite is designed to roll back and discard its work in that case - no harm done. But I suspect when you came back again on second attempt, that second attempt was much slower, because… SQLite was cleaning up from the first partial attempt? I’m not totally sure.
But yeah, I think that’s the most likely explanation. An unfortunate series of events and misdirections.
Oh and the situation that could have led iOS to think it needed to show that fake placeholder screenshot for too long, that’s been fixed in the new build 47. So thankfully that totally misleading situation shouldn’t happen again!