I'm trying to setup some additional env variables for openproject docker like it's described here: https://www.openproject.org/docs/installation-and-operations/configuration/
But whatever additional env variable I add to the docker-compose.override.yml is being ignored. For instance:

version: "3.5"

x-op-app: &app
  environment:
    OPENPROJECT_SMTP__ADDRESS: "mail.gmx.net"

does not work, but adding it to the original docker-compose.yml does work.
Weirdly, setting a different option for db in docker-compose.override.yml works fine:

version: "3.5"

services:
  db:
    user: "1009:1009"

This successfully runs the database user as 1009 and all files created by that service are owned by UID 1009.

So docker-compose is picking up the right file, it just seems to ignore env variables for openproject for some reason.
My system is:

docker --version
Docker version 20.10.12, build 20.10.12-0ubuntu2~18.04.1

docker-compose --version
docker-compose version 1.21.2, build a133471

Any idea why the env variables for x-op-app aren't picked up?