Create new supplier.
Assign a default project tag for a project that doesn’t exist. It seems to allow me to create a new project.
Create another new supplier, the project still doesn’t exist.
Open first new supplier, project seems to exist.
You can set a default tag at the supplier level but I suspect the project behind that tag only gets created in the database once at least one invoice or purchase has been assigned to it. If you created an invoice for supplier 1 then you’d probably see the tag available for supplier 2.