Offline persistence after restart app

[dzuncoi]

Currently I had an issue about offline persistence (on iOS)
First I call an update request (while offline) like

firestack.databse.ref('some/ref')
.set(myData)
    .then(() => {
      resolve('Synchronization success');
    })
    .catch(err => {
      reject('Synchronization fail');
    })

Debug logs show me that Firebase had created some locally ref to store my data. Then I completely close (double click Home button > swipe app away) then reopen app, Firebase should sync my local data to remote, but it doesn't.

I'm using branch master, not v3.
Is anyone facing this problem?