Flutter / Firebase push notifications not fired when app is closed

I am using firebase messaging to send notifications to users in flutter application, and everything is working fine when I am sending notification from postman (notification always shows), but then when I want to send the same notification from my backend server, notification not showing on device. On server logs I can see that notification was send with success like this:

{
    "multicast_id": XXXXXXXXXXXXX,
    "success": 1,
    "failure": 0,
    "canonical_ids": 0,
    "results": [
        {
            "message_id": "XXXXXXXXXXXXX"
        }
    ]
}

So it means it was correctly send to messaging services, but push notification not firing on device.

Sometimes when I open app (going to foreground of application) then notification somehow shows, but I need users to see notification always, especially when their app is closed.

I am sending notification with high priority so this is not an issue.

Any help, what could be potential issue with this?

My request to FCM:

POST: https://fcm.googleapis.com/fcm/send Authorization: key=xxxx_MY_KEY_XXXXX Content-Type: application/json

{
    "to": "xxxxxMY_FCM_TOKEN_xxxxxx",
    "mutable_content": true,
    "content_available": true,
    "priority": "high",
    "data": {
        "content": {
            "id": 106,
            "channelKey": "remote_chann",
            "title": "Hello",
            "body": "test",
            "notificationLayout": "Messaging"
        }
    }
}