Hello Jens,
This is the correct behavior.
In your example, the service item is with type=service-fix price in a contract and invoice schedule is maintaine. In this case, once the contract item released, the Customer Contract Invoice Request will be 'processed' and in the details table, you will see the contract item status is Not Relevant. Tis means it is not relevant for invoicing. This is becuase the invoice will be generated as per the scheduling.
If there is no invoice schedule made to the service-fixed price contract item, then, once the contract released. Customer Contract Invoice Request will be 'To Be Invioced', you can invoice it immediately.
It depends on what service type you selected, wether invoice schedule involved, what type of the document (sales order, service order, contract, ect). With different scenario, the invoice request based on the source document will be generated accordingly.
Kindly test it in your testing system, you will get better understanding.
Thanks,
Adie