EMQX_MODULE__SUBSCRIPTION__3__TOPIC doesn't work

[bestming8]

BUG REPORT

Expected behavior

in /opt/emqx/etc/emqx.conf, there will have some config item like
module.subscription.3.topic = m2m/%u/+/+/+/#

Actual behavior

there aren't any

Test code

docker run -d --name emqx -e EMQX_MODULE__SUBSCRIPTION__3__TOPIC=m2m/%u/+/+  emqx/emqx

EMQ version

3.2.3

Docker version

Which docker-engine version?

Docker version 19.03.2, build 6a30dfc

System

Ubuntu 18.04.3 LTS