I'm working with Firebase Cloud Messaging, and what I've discovered is that after sending notifications through CLI like this:
curl -H "Content-type: application/json" -H "Authorization Key" -X POST -d '{ "notification": { "title": "Title", "icon" : "ic_notify", "sound": "default", "body": "Body" }, "data": {}, "to" : "FCM Token"} ' https://fcm.googleapis.com/fcm/send | jq .
Out of 7 notifications, my device only received 3, so the first 4 notifications got lost even though the response was successful.
After that the notifications are received normally... I've tested also with a real device and also with a simulator, same result.
Thanks for the help!