VMware Cloud Director 10.6.x
The custom role was missing "Administrator control" rights.
The "Administrator control" right allows users to manage any objects owned by other users inside an Organization VDC.
Add the "Administrator control" right to the custom role. For additional information visit the documentation at Edit a Custom Tenant Role
To compare rights in the custom role to a default role available, refer Rights Included in the Global Tenant Roles in VMware Cloud Director