FCM behavior when offline for iOS

I'm having trouble figuring out how FCM behaves when I send a notification to an iOS device that is offline (e.g. in airplane mode, or turned off).

The documentation on the time_to_live property mentions that Currently, time_to_live is not supported for notification messages on iOS. but doesn't provide an explanation of what is done instead. I've tried testing it, and it seems like a push notification sometimes goes through, and sometimes does not, regardless of what I set the time_to_live property to be, although I'm not sure if that's due to throttling or something else happening on the FCM side.

Relatedly, I can't seem to get the delay_while_idle property to work on iOS, although the documentation does not explicitly mention that it isn't available for iOS - notifications that are sent while the phone is asleep still wake the phone, even if I set delay_while_idle to be true.

Does anyone have any insight into how this is supposed to work?

The time_to_live is AFAIK applicable to both Android and iOS. However, since the process of sending the message to iOS devices for FCM goes like this:

App Server > FCM Server > APNs > iOS device

It is safe to say that only the FCM Server makes use of the time_to_live, as per it's description:

This parameter specifies how long (in seconds) the message should be kept in FCM storage if the device is offline. The maximum time to live supported is 4 weeks, and the default value is 4 weeks. For more information, see Setting the lifespan of a message.

Looking around, the behavior for APNs when sending to offline devices is (from the Apple docs):

Apple Push Notification service includes a Quality of Service (QoS) component that performs a store-and-forward function. If APNs attempts to deliver a notification and the destination device is offline, APNs stores the notification for a limited period of time and delivers it when the device becomes available again. This component stores only the most recent notification per device and per app. If a device is offline, sending a notification request targeting that device causes the previous request to be discarded. If a device remains offline for a long time, all its stored notifications in APNs are discarded.

As of now, delay_while_idle is now deprecated.

The way I know that you could do to wake up an iOS phone (online/connected to a decent network) is to simply set the priority to high.