Order Splitter for WooCommerce has a variety of features to split items and options to handle the main/parent & splitted/child orders. One of these options, Hide parent order after split on thankyou page.
User can hide parent order on thankyou page with this option. Main/parent order can be displayed on thankyou page with condition. For example, we need to display parent order when some specific item(s) is in the parent order. To achieve this, add meta-key wos_force_display through custom fields in specific item(s).
Whenever an item with this meta-key will be in the parent order, parent order cannot be hidden on thankyou page even if you have turned on Hide parent order on thankyou page.
If you cannot find custom fields section on product edit page, make sure you have turned them on from screen options.