Okay, that is disappointing but seems to be a limitation of the gateway
Thing changed trigger is of no use to you - if the Thing says online when it isn’t, it will still be online when the bulb comes back and reports. No change occurs.
There isn’t any Thing-update-to-same-value trigger function, so far as I know.
EDIT - current docs suggest
Thing <thingUID> received update
is a valid trigger. I don’t know what OH version that arrived in.
You’re sure you don’t get an Item update at switch on, even if its to the same level?
I’m assuming there are no periodic status updates, which would allow you to use expire binding.
I think you are goosed here; if the gateway doesn’t make periodic reports nor does it report offlines, you’ve got no way to tell if a bulb is really offline or is just doing what it was last told to do.