@nprofile1q... that's an apple restriction: pushes to offline devices aren't reliable and nearly always get lost. Writing a second message to trigger another push, once the iPhone is online again, will always wake up monal and allow it to retrieve both waiting messages.

Side note: in siskin/snikket iOS, the notification for the first message will be lost and you won't get a notification for it even if you get a notification for the second one. Monal is different (see above).