To resolve this you have to go into settings and then system and reset to default. After a short delay it will return to the normal resolution.
What happened is that after installing Arctic Zephyr Reloaded everything looked good. At this point I wanted to see how the ProjectM visualization looked while playing music. During the steps to get to that I hit the full screen button on my tablet which is set to control Kodi running on the chromecast. Nothing happened so I hit it again. This was hit at the wrong time. If you know anything about how Kodi works on windows and Linux if you press the full screen button on yatse (the remote control for Kodi) it will either put it into windowed mode or it will put it in full screen mode. Also, if you are playing a song from a file (rather than from the library) and when it starts you can press that button for full screen and it will make kodi show a nice screen with the artist, song, visualization, etc. In products like LibreElec when you do this since there’s actually no windowed mode it will still try to put it into windowed mode. That can mess up your display showing only a tiny square with the whole of the program. If you press it again, lucky you if it goes back.
So, what happened was because I went to put it into full screen for music playback and it didn’t respond I quickly pressed the button again. This forced it into windowed mode (yes, on the google chromecast) and then set the resolution to something like 720×480. Going into settings then system then reset to default set it back to full screen mode and that set the resolution back to 1920×1080.