I had this problem when I started using Streamlabs and managed to fix it. Streamlabs had an update that messed with some of my custom settings for some reason and after I fixed everything the black bars were back I'm using a 1440:900 monitor, and I know twitch prefers 16:9 resolutions but i didn't have this problem before.
P.S I don't know how to post a Log file with streamlabs.
P.S I don't know how to post a Log file with streamlabs.