I have the MiLight binding running on the testing 2.4 branch although the same issue was there when on Stable.
Basically one of my zones has one bulb, another two bulbs and a thrd zone has four bulbs. I grated a group called all lights and add all the zones to it.
When a event is triggered it sends a on command to the group however quite often a bulb and multiple bulbs don’t recieve the command. I could be one bulb from the zone that has four or it could be one bulb from multiple zones.