I can think of a bunch of ways. 
The first thing to remember is that smartthings is a mostly cloud-based system, and that introduces a bunch of interesting possibilities both good and bad.
The status is changing in the cloud, and it being changed at the time that the message is sent that should cause it to change. But that doesn’t mean it actually is going to change down here on earth. So the status being out of synch doesn’t bother me at all, like I said, there’s all kinds of places where something could go wrong and that would happen.
So now the question is what’s going wrong. Could be a time out issue, could be a local network issue, could be an RF interference issue, could be a local siting issue. Or it could be a defective device.
Since smartthings doesn’t give us as customers any network diagnostic tools, we have to go back to “observe and record.”
If you swap the position of the two bulbs, we can eliminate a couple of things potentially. I don’t know exactly which is which but for now I’m going to say that in the dual fixture the left Bulb works and the right bulb doesn’t.
Swap them.
If the left Bulb (Which was previously the right bulb) still works, we know it’s not the end device. If the right bulb (which was previously left bulb), doesn’t work We know it’s something about the siting and we can start investigating that.
If the left bulb (which was previously the right bulb) Doesn’t work but the right bulb (which was previously left Bulb) does work, Then it’s not the siting. It could be a bad end device. But it could also still be the network. Or the cloud.
The next thing I would do is a network heal.
But it’s just a matter of trial and error, observe and report.