I have never heard nor seen that MIGO splits an outbound delivery.
A delivery split happens in the very moment when a delivery is created. You eventually have 2 different batches for the line item of your delivery.
I would be interested to see the delivery, its document flow and the change history for the item, this could eventually explain why SAP gets to the 49
As a general advise, you could avoid all these rounding problems if you would order and deliver in the base unit, it is a STO, business between sisters, sharing the same material, using the same base unit, why do you need an order unit in that case?