As I said, it’s the Stygian Commander Cuirass the one that doesn’t work correctly when riding a horse. The one you’re showing in the video is the Aesir Chieftain Breastplate, which does work in this case.
The bug both armours share is that when equipped on someone else, thrall or player, it looks stiff until they’re very close to you.
Also, I now have my settings in cinematic and it didn’t fix it.