Expose system policies to Chromium flatpak 

(
  sys_policies="/etc/chromium-browser"
  ext_point_id="org.chromium.Chromium.Extension" # Chromium
  #ext_point_id="io.github.ungoogled_software.ungoogled_chromium.Policy" # Ungoogled Chromium
  ext_point="/var/lib/flatpak/extension/${ext_point_id}.system-policies/$(uname -i)/1"

  sudo mkdir -p "$sys_policies"/policies/{managed,recommended} "$(dirname "$ext_point")"
  sudo ln -s "$sys_policies" "$ext_point"
)

References