I tracked it down to a couple blacklist lines I was using to get the package size smaller.
- …/…/…/Engine/Content/Slate/Fonts
- …/…/…/Engine/Content/EditorMeshes/ColorCalibrator
I’m not sure which of these was breaking it, but the first wasn’t even the cooked directory. I had copied these from an earlier project. It’s strange that blacklisting the second would have had any effect on the video.
We have since given up on making the package extremely small anyway, so I just removed them and now it’s working.