Not sure if this is a problem
But just did a default install on a factory wiped Home Assistant green.
MQTT v6.4.1 which support MQTT protocol 5.0, 3.1.1 and 3.1.
The default install of zigbee2MQTT v1.40.2
Which by default is set to MQTT V4
Not sure I this is causing the Device/Settings bug where any changes to Device Settings is automatically removed before you can press the Save button.
The ONLY way to set zigbee2MQTT correctly for the MQTT is in the zigbee2MQTT configuration.yaml file in the MQTT section.
mqtt:
server: mqtt://192.192.192.192:1883
user: xxxxxxxxxxxxxxx
password: xxxxxxxxx
version: 5
You can change it in the zigbee2MQTT UI Settings/MQTT/Version but it is not retained and automatically returns to v4.