Why doesn't the "Difference" blending mode work from inside a precomp?

Hi!

I'm having a problem with the "Difference" blending mode. Basically I have a picture with the Normal blending mode, and on top of the picture, I have a video with the Difference mode, and it shows the difference between the two in the comp preview window and everything is all well and good. When I precomp the two layers, the main comp preview window now just shows the picture layer, it's as if the video layer had vanished. If I open the precomp, both of the layers are still there and it shows the difference between the two layers correctly in the precomp's preview window, but if I go back to the main comp, it still doesn't work.

If I render out the precomp, it doesn't render the video layer either, it just renders the picture.

Sorry if the explanation is a bit confusing. Can anyone help me out?

IOW, the Preview in your PreComp looks as expected but the render produces something else?

Other than what Walter has suggested, you should also post screengrabs of your precomp's Timeline and the 'final' comp's Timeline.

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