If each frame in the media contains the exact 1920 × 1080 pixels ...
This image has the same number of pixels on the top and bottom half, but you can probably see the bottom half looks worse. That's what lower bitrate does. It's like turning up the compression on a jpg -- you are not getting the exact same pixels, just the exact same image size.
I've tried a bunch of karaoke software including trials of the commercial products. So far my favorite for house parties is Karafun for Windows. You need to dig up the v2 edition. They REMOVED the ability to play your own files in the new v3.
https://www.karafun.com/karaokeplayer/
Karafun isn't perfect, it can act up if you have a large library... Even freezing or crashing. But that generally happens at startup or shutdown, not during use. Imperfect as it is, it's still the best I have found.
We do serious parties, if I could pay someone for awesome karaoke software I would totally do it, but all the commercial versions are really hateful to use. (I guess I should take another look, it's been a couple of years.)
Edit to add:
I just re-tried Kanto Karaoke, it looks so promising... But as with every app I have tried it has some usability issues that are fairly devastating, for me anyway. I think it is worth a look if you are exploring the free and affordable options, though.