It appears in the past someone has deleted our 'Do not delete' Site Administration Group, and Sites Content groups.
This means that when viewing the Site in the list, the number of groups associated with it are zero:

We have created a group with edit permissions and have no issues editing the existing site, adding new pages etc. We do not share any content at this stage, so in reality the lack of a 'Sites Content Group' is not affecting any functionality or management of the Site itself - but there are no groups in the 'Manage Groups' section so I'm concerned this may be an issue in the future if we did end up needing it for something. Clicking either of the highlighted buttons in the image below does nothing.

We use Enterprise 10.8 with DEV/TEST/PROD, and luckily in PROD the arrangement is all set up correctly, so I am just looking to fix our TEST environment to ensure that going forward any new developments will work properly at wont require a rebuild at an inconvenient time.
Is there any way to reassign the Admin Group and Content Group to the actual site itself without having to recreate the entire site? We have updated the JSON in ArcGIS Assistant to assign the correct groups in the 'Description' view of the parent app portal item but this didn't appear to assign the groups in the App view unfortunately.
If there is a solution in a newer version of Enterprise then I am still keen to hear it as we are expecting to upgrade shortly so I can add this to a post-upgrade to do list that is ever growing.