Might be a cabling issue as well. Changing HDMI port on the TV might help if the contacts are dirty. 4K requires more bandwidth and the cable might not be able to keep up.
I had weird flickering with a different video app that was caused by Android system WebView. I had to uninstall updates to it and clear data.
Hi, u can go to the device settings menu, go to display, update your resolution option, and yes, do check on your HDMI port, is it dirty or not, last but not least, force stop disney+ and clear the data, hope this can help u out