PO number field in Project Contracts

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In Project Contracts, there are PO number field in both Contract header and Contract line -> Billing Tab.

PO number field is free text user enterable and the hint on the field shows "PO number is used for reference".

When I Create a contract with multiple lines (each line have the same bill set number) and provide different PO#s at each contract line billing tab,(No PO# at contract header billing tab), Validation fails with below error:

Error The invoice format attached to the bill plan isn't grouped by contract line and the contract line is either item based or has a purchase order number.