Hello Aafke, and thank you for posting to our community.
If a Shopify order is split and a portion is shipped, ShippingEasy will update the Shopify order status as "Partial", with the respective tracking number. Once the order is fully shipped, ShippingEasy will update the status to "Fulfilled" and send the remaining tracking information.
Any split order portions left unshipped in ShippingEasy, will no longer receive updates from Shopify. This means if the remaining order is fulfilled outside of ShippingEasy, the remaining unshipped portion in ShippingEasy, will not update accordingly, and instead, will need to be cleared manually.
For future reference, you can find this and other Shopify specific related information in the link below.
Thank you and happy shipping!