Thank you. With this new knowledge in mind, I now know exactly why my packs aren't working. Because I didn't buy them on the legacy Xbox One version, and also why the Halloween Mash-Up is working perfectly fine, because I did buy that one on the legacy version. I'll buy the packs that aren't working again on the legacy version and update you if it makes a difference.
I mean "Not work" as in, whenever I try enabling the texture packs they always give me an error message saying they failed to load, no matter how many times I try again, it always just says "Pack failed to load". This is especially annoying for the Festive 2016 mash-up, as I wanna re-live 2016 memories, but both the textures and the world that comes with it always fails to load.
Two days later, still isn't working. This might need to get fixed in a future update.