Hello,
That is standard system behavior and working as per design. There is no issue, no bug, no problem.
If the batch code is mentioned on the main line item it is no longer possible to create a batch split. That is one reason the system will create a separate batch split line item during partial delivery quantity updates. Another reason is the fact that the full quantity of any given batch may not be known while the transfer orders are still open (when the batch is not already determined of course). The quantities picked are added to new lines to avoid promising the full amount to specific batch codes before final pick confirmation. The picks are separate updates to the delivery and segregated by lines. SAP delivery line item quantities are not fluid, meaning they do not change systematically. The total of a specific batch picked is not aggregate when separate transfer orders are confirmed.
If you look at a batch split in your scenario you will see a delivery quantity assigned to each batch split line, you will also see the picked quantity for each batch. The delivery quantity on each separate line is the thing that is not fluid, it does not change therefore for each separate transfer order confirmation of a batch (unique or not) you get a separate delivery quantity and batch split line. SAP does not add the separate picks and include them in an existing batch split line (increasing the quantity on a line that is already "C" "C" (picked/packed complete) that's just the way it works brother