It's frustrating, but you get used to it. It's really common after a feature to get a few weeks of unsubs. The thing is, that the readers on the app probably aren't even aware you, the creator, get notified of subs, and they likely don't even think that you might pay attention to subs and unsubs, so they use subs almost like a bookmark. They see your thing in the listings, drop a sub on there so they don't lose it, then give it a shot later and... maybe they decide it's not for them after all, so they unsub to keep their feed clean.
It's almost never malicious. A large proportion of subscribers, especially when you're hitting larger numbers, aren't regular readers. That's just how it is. Some of them are a "dead sub" and others unsub.
Ultimately, hey, at least something about your work or marketing caught their eye and made them give it a shot, so it's not all bad!