It actually has to be a very specific format, we’ve found 1920x1088 (yes, 1088 pixels high) to be the resolution to work, with .mp4 @ 30fps, 48khz sampling rate AAC audio.
Good luck trying to use Adobe software to push out an .mp4 these days btw. This is a ridiculous bug that’s been in the engine for YEARS. I hope this helps someone trying to get the intro video working a few days before publishing their game, since it’s really hard to figure out and there’s literally one post on the Internet with the correct resolution.