When I look on videos in the theater every thing is a bit slower and it looks like all of my shots are out.
anyone knows how to fix it or why it happens?

There is no fix to the way Theater currently works. It’s been known to have issues since release.

Yeah I wouldn’t be surprised if it is removed come Halo Infinite. I never liked it but it did cover me a couple times when I forgot to record a clip mid game.