Thank you for the quick reply rimelek.
You’re right, running ncdu with -x I now see that the files are only being stored on the external mount.
The files were originally moved manually over from the default volume location to the external mount.
However, looking back I think I probably forgot to delete the volume before running the changed compose file, since my primary drive ran out of space again when scheduled backups were triggered later on.
That, in combination with me not using the correct flag on ncdu caused me to think the setup was somehow incorrect (compose file was correct but lacked a delete) and at that point I deleted the volume, ran the compose file again and used ncdu incorrectly again (compose file was correct, state was correct but the signal from ncdu was incorrect).
Thank you for the help, you probably saved the rest of my Sunday!