Power BI Groups cannot be found in Azure AD

We have a similar issue here. Using Powershell Get-PowerBIWorkspace gives me all of the workspaces, which are a mix of "Group" (v1) and "Workspace" (v2) types. Of my (currently) 38 v1 workspaces, the Workspace ID matches up to the corresponding AAD Group GUID for 22 of them, but the other 16 seem to be headless. My suspicision is that the workspaces were created automagically during the creation of Sharepoint Online sites (or something similar), and they've now been deleted but the Power BI Artifacts were not tidied up along with them.

So how can we delete these workspaces? I am the PowerBI admin and it won't let me delete them as I don't have "workspace level" permission - of course I can't *GET* workspace level permission for them as the AAD group which i'd need to be added as an Admin to does not exist.