try

    systemctl enable --now xdg-user-dirs-update.service

and just to be safe I have a snippet that parses that file and exports the variables in my login shell like .profile

(fish in this example but would be similar for other shells)

    sed -nE 's/^([^=#]+)=(.*)/set -gx \1 \2/gp' <"$XDG_CONFIG_HOME/user-dirs.dirs" | source