Thanks for the info, I found the answer:
This is a secondary cost transfer so will update the GL via the
Real-time Integration of Controlling and FI. As the secondary cost
elements used in the transfer don’t exist in the GL we have to derive the
required GL accounts. This is configured in the IMG in:-
Financial Accounting (New) Financial Accounting Global Settings (New)
Ledgers Real-Time Integration of Controlling with Financial Accounting
Account Determination for Real-Time Integration
Define Account Determination for Real-Time Integration