Light group status not updating correctly

I have two Hue bulbs, connected direct to my HA hub using ZHA, which I have added to a light group (as they are both in the same light fixture). Sometimes it all works as expected, but others the system shows the bulbs in the incorrect state (i.e. bulb is off but HA thinks one or both are one, which keeps the group on).

Any ideas where I should start looking?