Only One Site Level Term Group Allowed? ( type: SiteCollectionGroup ) Graph API & GUI Attempted

Put simply, I am unable to find a way to create more than one group in the 'Site level term groups' area within a given site's settings. Either via the GUI or via the Graph API options.

If I send a POST to the /termGroups endpoint it just modifies the current, sole, site level group.

The Graph API docs do not indicate any limitation, and neither do the SP docs that I've found (here, here, and here) ... just wondering if someone out there knows something obvious that isn't within my current realm of finding.