SAP Billing - Combine Billing for deliveries with different date
When using transaction VF04 or Billing (background), the date of the billing document (e.g. the current date) must be entered (In VF04 : settings, default data.)
In VF06 or background: variant with parametrization) to avoid an unwanted split due to the billing date.
This OSS notes is very helpful :-
11162 - Invoice split criteria in billing document
36832 - Invoice split in fields from the sales order
