I read a forum post that suggested the overnight automatic firmware update from the Hue hub may be the problem, but disabling this hasn’t improved matters at all.
I’m desperate for a solution, as this has effectively broken my smart lighting setup. I can’t afford to replace all the Tradfri bulbs with Hue, but am considering ditching the Hue hub and the last few Hue bulbs I have (unless they can be connected to a Tradfri hub?)