MQTT debug

I have a Sonoff module with Tasmota. The module works fine in HA but I’d like to extend the functionality.
I enabled Tasmota logging to level 4 (debugging) for both Weblog and MQTT.
In the Tasmota weblog I see messages passing when I do a button press on the module.
Unfortunately in the log of the MQTT Mosquito add-on I only see:

1615411691: New client connected from as DVES_102A50 (p2, c1, k30, u'mqtt_zigbee_user').

no details about the button press.

Is there any functionality I have to enable in MQTT to start debugging?

Try the mqtt dump service in Developer Tools /Services:

Open your Home Assistant instance and show your service developer tools.

Thanks, now I can see what’s going on