Just a quick correction, Sunshine isn't a RDP client. Sunshine is just a moonlight server which uses a completely different low latency protocol than anything like RDP or VNC.
What I suspect is that you are missing the codex needed to properly encode the video. What hardware to you have? How did you install Sunshine? Nobara is Fedora based so you are likely having an issue some fedora user has had.
Please give me a bit more information so I can help you.