If you look through https://store.gx.me/mods/?tagAlias=desktop,splash-screen, mods with splash screens seem to only be created by the "Opera GX Official" studio. That might mean that regular users can't publish splash screen mods at this time. The Schema 2 documentation isn't even published online as far as I know. It seems everyone just learns from existing mods.
The only other thing I can think of is if your splash screen video is too long. Make sure it's no more than 5 or 6 seconds.
I really doubt it's an issue with the mp4 format, but you could try webm (vp8 or vp9 + vorbis or opus) instead just to see.