I have the same issue. I have a Dell Vostro laptop connected to a Dell 3100 USB3.0 Dock (with a LG 34" Ultrawide 34BL650-B monitor), running Windows 10 ver. 21H1 build 19043.1165) . I noticed when I disconnected from the dock, my iTunes videos (using v 12.11.4.15 for Windows) displayed as normal. I made sure to download and install the latest Dell firmware/drivers for the 3100 dock. Still iTunes videos pixelated as you describe.
I decided to revert back to the previous version of iTunes for Windows (12.11.3.17). Before doing so, make a copy of your iTunes .XML library file, by clicking on FILE, LIBRARY, EXPORT LIBRARY. You'll need that to rebuild your library.
This video describes the steps to revert:
https://www.youtube.com/watch?v=m9TWd7ErBcY
After uninstalling 12.11.4.15 and components as described in the video, install your preferred version. To import your library .XML file, move (or copy) it into C:\Users\<USERNAME>\Music\iTunes, and click on FILE, LIBRARY, IMPORT PLAYLIST.
Hope that helps.