That is my guess too. If so, I will just patiently wait I am trying to rule out that I have done something wrong in the setup.
The lamp is added in Homey as tunable bulb. MQTT Hub then creates it in HA via auto-discovery. It shows the tunable slider in HA.
But the capabilities/properties broadcasted by MQTT Hub as shown in my previous pictures are only onoff and dim.
The lamp can be fully controlled from Homey interface (onoff, dim, colortemp). But MQTT Hub only sends onoff and dim commands. If I adjust the color temperature, nothing is sent to MQTT.
From HA I can control onoff and dim, its sent via MQTT, picked up by MQTT Hub and Homey sets the lamps accordingly.
The problem is that when I adjust the color temperature from HA, HA sends the commands via MQTT, hence creating that topic as shown in my MQTT-Explorer picture. But it is not acted upon by MQTT Hub and Homey is not adjusting the color temperature.
So the strange thing is: MQTT Hub sets up the tunable bulb correctly in HA. But does not react upon the color_temperature adjustment.