I also have this problem with a Roku Ultra, model 4800X, serial # X01000F73GV3, software version 10.5.0 build 4208-C2. The video that's supposed to be played is too small and in the upper left of the screen, but the controls (progress bar etc) appear on the entire screen so it's not that the Roku can't "access" parts of the screen. I've replaced the HDMI cable with a high quality one but this still happens almost every day, and on all different channels. (I also have seen an HDCP error message at times, but I think the new cable may have alleivated that aspect of the problem.)
I bought this unit to replace an almost 10-year-old Roku that never gave me any trouble, and I only replaced it because the old one couldn't run the HBO Max app. I'm disappointed that I constantly have to restart and fiddle with it. Can someone from Roku contact me?