I can't say I've used the dusk.dawn event to turn on a light before, but I have used it to email me when states changed when I was trying to get a sense of its behavior. What I noticed was that my various motion sensors would change dusk.dawn state multiple times a day ... maybe 5 to 8 times randomly. I also noticed that it would sometimes trigger the same state multiple times. For instance, it would trigger on and then 30 minutes later trigger on again. In the end, I can't say whether or not this explains your situation, but I do think this may be another variable to consider.
As to what you are trying to accomplish is it possible to use programs in combination here? If so, you could have one program turn on the light from dusk and off at dawn. A second program could look for motion events during dawn til dusk. Of course this would mean the ISY is triggering the scene and not the sensor. Not sure if that delay is okay for your use case.
Also, from other threads I have come to understand that 'As Motion Occurs' will send continuous on messages as it sees motion which doesn't seem useful for this case. The 'After timeout only' setting should work and will generate less traffic and battery drain.