@Tom-Elliott to further iterate and give some more specific information.
Unicast worked because while the order was not “proper” it pulled the partition number from the iterated item. Example /dev/sda1 would look for file d1p1.img. /dev/sda10 would actually look for and use d1p10.img.
In multicast this iteration happens but the data is sent by the server.
It did not scan for a particular file.
So in your case the commands were sent in expected order.
udp-sender would send in order
d1p1.img, d1p2, d1p10
It sent the commands in that specific order. The partition receiving the file was not matching the file it was receiving. /dev/sda1 would get d1p1.img properly but /dev/sda10 was getting d1p2.img. Hopefully that helps make sense of the problem.