@george Thanks for checking. That's really strange. At first I thought the issue was associated with the monitor—some HDMI compatibility glitch—but was able to get the same behavior across three different televisions. I've done it with both the 20230424 and 20230306 Ubuntu 22.04 images as well as the new 23.04 image. I've even installed the image on both the SSD care and the built-in eMMC.
I've also kept syslog and dmesg logs open (on an SSH session) and I'm not seeing any messages that make sense. All I know is that the instant I hit the OK button to log out or restart or shutdown, the screen freezes and the mouse pointer doesn't move. But it doesn't happen with the built-in default "coolpi" account.
If anyone has any theories or ideas of what I might capture to further diagnose, let me know.