If the project is marked as complete (status of the project), it is not shown in the invoice selection during invoice creation. We complete the project and then bill it. In most of the projects the invoicing is done when the project is completed. unless it’s an on-going type project.
I can replicate the issue in test account as well. Would you mind creating a github issue for this with the use case.
For now you can keep the Project open and mark the % completion as 100 when Sales invoice is to be raised.
Created the github issue.