It might depend on how the movie was encoded, and what exactly the GPU fills the out-of-frame image data with. In datdudes case it was apparently the treacherous "empty green" which is basically the value 0 for chroma. Chroma upscaling would then read some of these green pixel in the upscaling process, creating a faint green echo. Reading those can't really be prevented cleanly, as any reasonably fancy upscaler will read an area to determine the pixels, so the only solution is to cut them off, but that is mildly costly, especially when you are already barely making it.
In any case thats the theory whats going on, its in itself logical, but would need confirmation its fixed for datdude now.
Even if the outside pixel were black though, you might get a slightly darkened line along the bottom edge or such, which still wouldn't be great - even if much less obvious.
An option might be possible, but I'm weary about adding one thats not exactly quality, but a bug.
