[estel]

When the push notifications hit FCM, Firebase do not guarantee delivery of those messages to clients (usually iOS or android devices). There are quite a few reasons that FCM/APNS might fail to deliver a message, so applications almost never have functionality depend on them.

As you say, you might not get the notification pushed to the device, but you should still see the message if you open the messaging app as normal.

[jhgg]

This is indeed the case. Our real time system is outside of firebase and APNS and it handles the actual real time updates of chat state once the app is launched. We also have a delivery system that accounts for network cuts/switches and the like.