@tizzyapunkt this issue has been filed with the team and should be fixed in the forthcoming release. In the interim Virtual Foreign Keys can be created programmatically via the API. Schema operations that can be done via the UI can be done programmatically via the API. For the full detail on the schema resource and how it can be viewed and managed via the API go here.
Alternatively, if you'd like to checkout the git tags for a previous version where this worked, looking at 2.6.0 and earlier iterations this should be working.