Any copyrighted music that you don't have permission from the creator/owner to rebroadcast can possibly be muted in your past broadcast videos. Even if you do have permission, it'll get muted and you'd have to appeal it to get it unmuted, showing proof you have the rights to rebroadcast whatever copyrighted music you're playing.

Twitch is using a software that detects detects audio content, which is how it ends up muted. I don't believe it matters if you're partnered or not. I believe in one of my game streams like 30 seconds got muted because a song in the game was being played and it picked it up. So, while you're streaming now it's fine, but the video recorded afterwards might be messed up.