My bot can send a response in multiple messages. I use the delivery callback to know that a message is delivered and the next can be sent. (https://developers.facebook.com/docs/messenger-platform/webhook-reference#message_delivery)
It works great except that sometimes I didn't receive Facebook's callback. I see no exact schema, for the exact same conversation it can works perfectly or miss a callback.
Have you ever noticed this problem ?
Opened a bug on Facebook plateform, answer is 'well do not rely on this delivery callback as it works mysteriously'
Update
The day after I closed this bug, new callbacks have been available on Messenger Platform, and the echo callback works exactly as intended.
Documentation of Echo callback here