Oh wow Scott,

I forgot all about this. I noticed this same thing in a theater recently where the intensity dipping during crossfading looked significant. I was crossfading intro clips into looping clips of the exact same images and then back out again. I though it was the content until I put the same image on 2 layers and tried again.

It's almost the whole range from 30% up to 80% and it looks like almost a 10%-15% drop. It doesn't happen in a Mix when "Flat Constant Aspect Ratio" is selected but for any other mid type it does.

What I actually just noticed when I opened the show file is that the whole mix actually goes slightly transparent for a moment.

What I mean is that if for example:
Mix 1 and 2 stacked.
L1 is full with a test patter on mix 1.
L2 & L3 on mix 2 at full
When I crossfade L2 to zero, I can actually see the test pattern in mix 1 coming through.

I actually forgot all about this until now. It's actually a very big deal for me too since one of my clients is constantly putting me in the position of crossfading identical content.