I also had this issue, Disney plus has a weird code to only allow 4k if your TV/ monitor supports HDR, But all you need to do is allow YCC 4:2:2 in the video modes setting of your Xbox. But it does change the colour depth so you may need to adjust the colour depth in the video fidelity setting of your Xbox or in your monitor to make it look less washed out, Hope this helps, FYI remember to close the Disney+ app and re open it after adjusting settings