How to enable MQTT Retained Events?

Hello, I receive data very slowly from end devices, so when my app connects to the MQTT server that Chirpstack updates, I want it to receive the last updated value. My codec already attaches a timestamp each time the data comes in from remote devices, so I can tell when the data was received.

I found this unrelated page that shows a setting in ./etc/chirpstack-application-server/chirpstack-application-server.toml called retain_events. I tried to set this value to true and restart everything, but it never retains the incoming data.
Also, as seen in the code here, the retain_events setting does seem to be supported.

What am I doing wrong?

Chirpstack application server version 3.17.2 on Raspberry Pi Debian 10

Thank you.