If you are experiencing issues with the brightness of your light group in the Philips Hue app or HomeKit UI, there are a few possible causes and solutions to try.
One possible cause of this issue is that the light group you are using contains lights with different brightness levels. When a group of lights with different brightness levels is adjusted in the app or HomeKit, the app or HomeKit will try to set the brightness of all the lights in the group to the same level. This can cause the brightness to appear to jump or spring back to an intermediate value briefly before returning to the desired level.
To fix this issue, you can try creating a new light group that contains only lights with similar brightness levels, and using that group instead. This will allow the app or HomeKit to adjust the brightness of the lights in the group more smoothly, without the jumping or springing back behavior.
Another possible cause of this issue is that the app or HomeKit is having difficulty communicating with the Philips Hue Hub. This can cause the app or HomeKit to display incorrect information about the state of the lights, including their brightness level.
To fix this issue, you can try resetting the Philips Hue Hub and reconnecting it to your home network. This will help to ensure that the app or HomeKit is able to communicate with the hub and receive accurate information about the state of the lights.