Thanks for your bugreport. I started on reproducing this but the current instructions have an isssue. The command "sudo sh -c 'cat /dev/null > kernel.efi'" destroys the current kernel (the good kernel). So trying to revert to that will always fail.

To simulate a bad kernel boot you will have to stop the shutdown after the refresh with "shutdown -c" and then use "sudo sh -c "cat /dev/null > /run/mnt/ubuntu-boot/EFI/ubuntu/try-kernel.efi""

With that I see that the try-boot fails and I need to power-cycle the system in grub. then it rollsback though:
"""
mvo@ubuntu:~$ snap change 6
Status  Spawn               Ready               Summary
Done    today at 10:41 UTC  today at 10:47 UTC  Ensure prerequisites for "pc-kernel" are available
Undone  today at 10:41 UTC  today at 10:47 UTC  Download snap "pc-kernel" (1021) from channel "22/stable"
Done    today at 10:41 UTC  today at 10:47 UTC  Fetch and check assertions for snap "pc-kernel" (1021)
Undone  today at 10:41 UTC  today at 10:47 UTC  Mount snap "pc-kernel" (1021)
Undone  today at 10:41 UTC  today at 10:47 UTC  Run pre-refresh hook of "pc-kernel" snap if present
Undone  today at 10:41 UTC  today at 10:47 UTC  Stop snap "pc-kernel" services
Undone  today at 10:41 UTC  today at 10:47 UTC  Remove aliases for snap "pc-kernel"
Undone  today at 10:41 UTC  today at 10:47 UTC  Make current revision for snap "pc-kernel" unavailable
Done    today at 10:41 UTC  today at 10:47 UTC  Update assets from kernel "pc-kernel" (1021)
Undone  today at 10:41 UTC  today at 10:47 UTC  Copy snap "pc-kernel" data
Undone  today at 10:41 UTC  today at 10:47 UTC  Setup snap "pc-kernel" (1021) security profiles
Undone  today at 10:41 UTC  today at 10:47 UTC  Make snap "pc-kernel" (1021) available to the system
Error   today at 10:41 UTC  today at 10:47 UTC  Automatically connect eligible plugs and slots of snap "pc-kernel"
Hold    today at 10:41 UTC  today at 10:47 UTC  Set automatic aliases for snap "pc-kernel"
Hold    today at 10:41 UTC  today at 10:47 UTC  Setup snap "pc-kernel" aliases
Hold    today at 10:41 UTC  today at 10:47 UTC  Run post-refresh hook of "pc-kernel" snap if present
Hold    today at 10:41 UTC  today at 10:47 UTC  Start snap "pc-kernel" (1021) services
Hold    today at 10:41 UTC  today at 10:47 UTC  Clean up "pc-kernel" (1021) install
Hold    today at 10:41 UTC  today at 10:47 UTC  Run configure hook of "pc-kernel" snap if present
Hold    today at 10:41 UTC  today at 10:47 UTC  Run health check of "pc-kernel" snap
Done    today at 10:41 UTC  today at 10:47 UTC  Handling re-refresh of "pc-kernel" as needed

......................................................................
Update assets from kernel "pc-kernel" (1021)

2022-06-20T10:42:43Z INFO No gadget assets update needed

......................................................................
Make snap "pc-kernel" (1021) available to the system

2022-06-20T10:42:43Z INFO Requested system restart.

......................................................................
Automatically connect eligible plugs and slots of snap "pc-kernel"

2022-06-20T10:42:43Z INFO Waiting for automatic snapd restart...
2022-06-20T10:42:43Z INFO Waiting for automatic snapd restart...
2022-06-20T10:42:44Z INFO Waiting for automatic snapd restart...
2022-06-20T10:42:44Z INFO Waiting for automatic snapd restart...
2022-06-20T10:42:45Z INFO Waiting for automatic snapd restart...
2022-06-20T10:42:45Z INFO Waiting for automatic snapd restart...
2022-06-20T10:47:05Z ERROR cannot finish pc-kernel installation, there was a rollback across reboot


"""