Yeah I noticed recently the notifications are late. I would go into whatsapp to start a new chat with someone and new messages will appear which was sent some time ago. Read them, exit whatsapp then suddenly a few minutes later will have the notification for messages I've already received and read. Or sometimes my friends would send multiple messages at one go and I would only receive the last message's notification.
I think it's whatsapp server issues.
I think it's whatsapp server issues.