We will be addressing some issues around splitting orders + financial calculations for v3 in the next few of weeks. If you could add the specific endpoints you are calling and show the actual vs expected responses, that would be extremely helpful.
For v3 orders, financials are now automatically recalculated when orders are split by either of these endpoints:
/order/{order_number}/process_dropshipments
/order/{order_number}/split
You also mentioned creating multiple shipments for v3 orders via the v2 api, but that is not supported. V3 orders should be split instead of creating multiple shipments for them.
Only product_amount and grand_total are recalculated.
For shipping, discount, and tax, they are passed on to the first order and zero’d out on the next order. We want to include the other amounts in the future, but we will need to make some adjustments so we can properly include information about how those values should be split. Just evenly distributing them is not the correct thing to do in many cases:
shipping costs usually increase when splitting because it will require multiple shipping labels
discounts are often tied to specific products or groups of products
items may be taxed at different amounts
So for now, we expect people will need to make their own adjustments to those amounts after the split if they are important.