Reading the following docs: https://developers.facebook.com/docs/whatsapp/cloud-api/guides/experiments it appears that facebook is experimenting with 1% of users which causes message templates to not being sent to this 1%.

This is affecting our experience as many clients are reporting that they are not receiving emails. Can you please remove this experiment for message templates? is a horrible user experience to explain every time to the user that they didn't receive the message because of this reason and that they need to send a message first to our Whatsapp Business API number in order to receive it.