I have strange issue with some of my Hue lights after i ditched the Hue Bridge and paired them directly with zigbee2mqtt and Home Assistant.
Some of then, not all, only return the On state but with 0 brightness in HA.
I can set the brightness and the bulb responds.
But when I close and reopen the popup it displays 0% again.
When I check zigbee2mqtt the brightness value is updated.
Anyone else experienced this?
Managed to fix this issue after some digging.
Error message in HA was…
Invalid or incomplete color value ‘{‘brightness’: 175, ‘color_mode’: ‘xy’, ‘linkquality’: 96, ‘power_on_behavior’: None, ‘state’: ‘ON’, ‘update’: {‘installed_version’: -1, ‘latest_version’: -1, ‘state’: None}, ‘update_available’: None}’ received for entity light.arbetsrum_skarm
When I removed the Hue lights from the Hue bridge and paired them with HA through zigbee2mqtt they had no color_mode value. The value was set to ‘color_mode’: ‘xy’.
To fix this all I had to do was to set a color temperature for each light in zigbee2mqtt and that fixed this issue.
I am so glad I found your post! I was seeing the same errors in my HA logs, and I was observing weird behaviour in the Home Assistant UI where the brightness slider of my Hue lights wouldn’t match the actual brightness. The latter seems to be resolved as well after applying your fix.
I was in the same boat as you: I moved all my Hue lights over to Zigbee2MQTT after I somehow fried my Hue Bridge v2. I used it as an excuse to make 1 big Zigbee network instead of 2 smaller ones. Hopefully I can stick with it.
You probably saved me bunches of time, so thank you! 
Observed the same behaviour. Interestingly not with my hue bulbs, but with the innr bulbs.