OK, this kind of works. If I change my "default appdata storage location" to my cache location /mnt/vmachine_cache/ as shown above in my screen shot from previous post, then it will resolve the issue where for mariaDB it considers those to now be 'internal' volumes.
For me this is ok (because cache is basically my default appdata location), but for other people their "default appdata storage location" might be /mnt/user and they only changed it manually for mariaDB (or similar) database performance. In that case their situation would be what I've shown in the screen shot in previous post, that is their 'default appdata storage location' = /mnt/data but in their mariaDB (or similar) container they are pointing manually to /mnt/any_name_cache/ and based on my experience that would be considered 'external volume'