I attached Facebook Login into my iOS app written in Swift, and for the first time after implementing stuff, I get dialog with question for opening Facebook app (as usual in iOS), I agreed and everything works fine.
Then I wanted to test it again but now I don't get any dialog and every time SafariViewController opens. I reinstalled app, but it didn't help. It's weird, because I agreed for first time to open app.
I also have this in .plist as described in docs:
I think it is not a big deal for you as long as the app works.
Sometime our iOS device will open Facebook app (if it is installed), sometime our iOS device will open Safari, or even if you already logged in and in a certain time, you can log in without asking any permission (which you did before).