Okay, so I looked into this a little bit, as you're not the only person to report it (see below), and I was once again unable to reproduce the problem on my end. It works fine in my test project, with images and with videos.
The problem, as it turns out, is that some update to the core files in the past made it no longer work anymore. I made a brand new test project and copy+pasted the same test files I was using in the test project into it, and it doesn't work. In fact, none of the splashes work, not even the images.
I looked into it a bit more and it seems that RMMZ has an entire splash scene that it defines in RMMZ_Scenes that it goes to before the title scene. I'm not sure what this scene actually does, though, as I don't see any options in the database to use it. Anyways, I managed to quickly fix the issue, as my plugin aliases the function to call a scene, checks if the current scene was Scene_Boot, and that the scene being called was Scene_Title, and then intercepts that call to send it to my pretitle splash scene instead. I just changed that to intercept scene_title or scene_splash.
The fix will be uploaded shortly.
~Ramza