Hi
Please check the below:
i. Payer
ii. Inco term
iii. Payment term
iv. Actual GI Date from Delivery
v. Shipping Condition
vi. Account Assignment Group
vii. Exchange rate
viii. Due to different item categories in a sales order
ix. Due to different billing document type
x. Due to Different referencing document types
Also check your copy control settings in tcode VTFA &, VTFL
If any of these get differs then billing will split.
Also please share the full screen short of split analysis and drill down the same.
Thanks.