It is consistent. If the current app was opened by another app (e.g. by clicking a link that opened Safari, or if you're in Safari and click a link that opens in another app, etc), the button will be there.

OTOH if the current app was opened manually by the user, it won't be there.

It is literally just one qualification, which doesn't even apply because why would the "back to previous app" button show when you're in the same app. In the meantime you haven't mentioned one inconsistency yourself. If you like discussions with made up arguments may I suggest you check out reddit?