Okay, I was wrong. The wallpaper layer actually is moving. It’s just moving very slowly.
As you can see, the mask is now at the top of the frame.

What would cause this slow movement? It’s almost as if it’s trying to pan over the entire image before I scaled it down. I originally brought it into the comp at 3072 X 2304.
I haven’t had any problems with large files in the past.