Well, I just tested that deep sleep theory. I hadn't touched my Oasis in a few hours and went into the bathroom again. I opened it and it goes to the waking up screen. I do see that screen often also, I do tend to take longer breaks too you know. One has to work and eat sometimes.
Anywho, I didn't notice a flash. It went to the screensaver with the 'waking up' at the bottom, then it went to my book. I have my light at 10 currently and I didn't notice it going higher than the 10. My eyes are very light sensitive by the way and a flash with higher than that on the kindle would blind me for a while. I read at 8 with a side lamp at night. I can't even read in total darkness.
So I still didn't notice any flash. Other than the black back to white flash all kindles have. But not a higher light level flash.
How high is that light going when it flashes? 20? 24? All the way? that would be very noticeable. You say its 12 as default? Maybe that isn't high enough from the 10 I had to to see it? 10 is too bright for me in the dark, yikes.
I grabbed my husbands phone one time in bed when he was showing me something. I couldn't see for half an hour after that. It was like looking into the sun. Ouch.
All I know is that I didn't notice any different in light level waking it up from deeper sleep. It looked the same to me.
eta: I have to correct something, I had the light at 9, not 10 today. I just checked.