Brightness of LED light strip not received via Z2M

I have got a LED light strip controller from Dresden Elektronik (Dresden Elektronik Mega23M12) with a light strip that has got separate LEDs for RGB and white.

In Home Assistant two entities were automatically created: one for the RGB LEDs and one for the white LEDs. For the RGB LEDs everything works perfectly.
But for the white LEDs HA does not get the actual brightness setting from the device. It is possible to set the brightness via HA but if it is changed outside of HA the brightness is not updated. In the development tools, I can see that the “brightness” attribute is either missing or has the value “null”.

The MQTT message sent from the device looks fine to me:

Topic ID is zigbee2mqtt/LEDStripArbeitszimmer/white
And this is an example of the payload: {"brightness":244,"state":"ON"}

In the MQTT debug information of this device I can see no problem:

Any idea what I could change to solve this problem?