Here is the additional information you requested:

The system is installed on SSD. This SSD with the same SW stack and RPi3b+ but using a different Sata to USB adaptor (instead of X820) works fine and supports reboot.

As you requested, I also tried a setup with just OS without Supervised HA - same problem on X820, reboot does not work.

Please note that I am using the latest version of 64-bit raspbian OS. I tried both full version and the lite version (without desktop) - the same problem exists when using X820.

Appreciate you looking into this issue, let me know if you need additional info.