It is in the Addons page - click on Zigbee2MQTT tile. Then click the configuration tab at the top.
There are 3 places to edit this - and this is the only one that seems to work.
- The Cog on the Zigbee2MQTT webUI - as you say resets immediately
- Edit it in the config file - updates (sometimes) but has no effect in practice
- In the Addons menu - that is where it works (after restart)