@Edent Keeping an eye on this as some of my candeo lights do this...
@Edent From reading through the most plausible and testable suggestion is that it's when zigbee drops/comes-back.. I guess that can be tested by forcing zigbee to go away and give it x minutes then bring it back up and see how it behaves? I'm not really seeing any other useful debug information though..
Mine are candeo bulbs and not using Hue.
@johnmclear yeah, looks like they either lost power or network connection and came back on.
Sadly my hub doesn't support sending them instructions to change that setting.