Too much deliver_status ! = "00"

We have a model of router 1, 3 endpoints, one coordinator.

 S2C Pro and All in API2 (FirmW#4055)
 EndPoint, router and  Coordinator have the same ST SP.
 only EP are fixed SM5 (which defines them as EP)
 the coordinator sends a packet to each EP one after another.
 all are in an office, without obstacle, we have an acknowledgment of receipt, ie non delivred TX status (deliver_status! = "00") goshawks 30%, which seems to us enormous.

Why the failure rate?

You are forgetting that a parent node can only hold either 200 bytes or up to two indirect messages in its buffer. If you try to send data to another node before the first polls the parent for the data, the oldest packet will be discarded in favor of the new one.

In addition, the TX status frame for a End device child is not sent by the child but by the parent node indicating that it has received the packet and will pass it on once the end device checks in.