Preferences are not written to core.conf, core.conf gets overwritten when container is started

Problems:
changes in the ui prefs are not written to core.conf
core.conf is overwritten with a default file when the container is started

docker-compose.yml used:

version: "2.1"
services:
  deluge:
    image: linuxserver/deluge
    container_name: deluge
    network_mode: host
    environment:
      - PUID=1000
      - PGID=1000
      - TZ=Europe/London
      - UMASK_SET=000 #optional
      - DELUGE_LOGLEVEL=warn #optional
    volumes:
      - /mypath/deluge:/config
      - /mypath/downloads:/downloads
    restart: unless-stopped

Changes made in the UI are not written to the core.conf.

I have checked that user 1000 can r/w core.conf in the host system and in the running container.
I also checked in the running container that /conf is mapped properly.

When editing core.conf on the host system and restarting the container, core.conf gets overwritten with defaults, so provisioning in this way is not working.

The logs show nothing suspicious:

deluge    | [s6-init] making user provided files available at /var/run/s6/etc...exited 0.
deluge    | [s6-init] ensuring user provided files have correct perms...exited 0.
deluge    | [fix-attrs.d] applying ownership & permissions fixes...
deluge    | [fix-attrs.d] done.
deluge    | [cont-init.d] executing container initialization scripts...
deluge    | [cont-init.d] 01-envfile: executing... 
deluge    | [cont-init.d] 01-envfile: exited 0.
deluge    | [cont-init.d] 10-adduser: executing... 
deluge    | usermod: no changes
deluge    | 
deluge    | -------------------------------------
deluge    |           _         ()
deluge    |          | |  ___   _    __
deluge    |          | | / __| | |  /  \ 
deluge    |          | | \__ \ | | | () |
deluge    |          |_| |___/ |_|  \__/
deluge    | 
deluge    | 
deluge    | Brought to you by linuxserver.io
deluge    | -------------------------------------
deluge    | 
deluge    | To support LSIO projects visit:
deluge    | https://www.linuxserver.io/donate/
deluge    | -------------------------------------
deluge    | GID/UID
deluge    | -------------------------------------
deluge    | 
deluge    | User uid:    1000
deluge    | User gid:    1000
deluge    | -------------------------------------
deluge    | 
deluge    | [cont-init.d] 10-adduser: exited 0.
deluge    | [cont-init.d] 30-config: executing... 
deluge    | [cont-init.d] 30-config: exited 0.
deluge    | [cont-init.d] 99-custom-scripts: executing... 
deluge    | [custom-init] no custom files found exiting...
deluge    | [cont-init.d] 99-custom-scripts: exited 0.
deluge    | [cont-init.d] done.
deluge    | [services.d] starting services
deluge    | [services.d] done.
deluge    | 17:11:13 [WARNING ][deluge.i18n.util           :83  ] IOError when loading translations: [Errno 2] No translation file found for domain: 'deluge'