:ro of external library is being ignored

I have the tree rows added to my docker compose-file under volumes: from the immich_server:
volumes:
  - ${UPLOAD_LOCATION}:/usr/src/app/upload
  - /etc/localtime:/etc/localtime:ro
  - /mnt/diskpool-1/plexmedia/fotos:/mnt/thorsten/fotos:ro
  - /mnt/diskpool-1/homefolders/username/fotos:/mnt/username/fotos:ro

The :ro is being ignored, I can delete fotos over the immich web UI.
What is the problem here?
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