Draft status is for generating invoices, without reference in the program.
Draft is a way for you to create invoice examples but without linking to references or they can serve you as a "test".
You must indicate this status before saving the invoice, so that it is not generated with reference.
If you want the invoice to be saved with a reference, you must access the invoice, click Edit and change the status to Pending. Once saved you will have your invoice referenced following the numbering of the document reference.