The problem seems to be:
$ initctl get-env -g XDG_DATA_DIRS /usr/share/ubuntu-touch:/usr/share/ubuntu-touch:/usr/share/ubuntu-touch:/usr/share/ubuntu-touch::/custom/usr/share/:/custom/usr/share/
Compared to a working, upgraded device: $ initctl get-env -g XDG_DATA_DIRS /custom/xdg/data:/usr/share/ubuntu-touch:/usr/share/ubuntu-touch:/usr/share/ubuntu-touch:/usr/local/share:/usr/share:/custom/usr/share/
The problem seems to be:
$ initctl get-env -g XDG_DATA_DIRS ubuntu- touch:/ usr/share/ ubuntu- touch:/ usr/share/ ubuntu- touch:/ usr/share/ ubuntu- touch:: /custom/ usr/share/ :/custom/ usr/share/
/usr/share/
Compared to a working, upgraded device: xdg/data: /usr/share/ ubuntu- touch:/ usr/share/ ubuntu- touch:/ usr/share/ ubuntu- touch:/ usr/local/ share:/ usr/share: /custom/ usr/share/
$ initctl get-env -g XDG_DATA_DIRS
/custom/