I think I identified the origin of the problem. It is related with field "Contact Persons ID".
When selecting a customer ID, the contact persons ID is automatically filled in with email address (not sure where this email is picked up, but inserting a different customer ID provides a different email).
The invoice creation / sending works fine when executing the step individually ("save and send test request".
But when testing the complete workflow, it goes into error, as the customer ID (picked up from previous creation step) is still mapped with the "old" email address (field "contact persons id") which seems to be static; hence the contact person is not found:
see the actual contact person ID, which is different from the one sent in the workflow step:
I have also tried to replace the mapped email with manual insertion of contact person ID from previous step, but I then get a message saying JSON is not well formed.