I could be emitting complete hot air here, and if so caker will correct me --
'reboot' and such in Linux will communicate to the kernel that a warm reboot is to be attempted. In our environment, there's no such thing. When you issue 'reboot', your guest domain goes away, and Lassie will jump in and issue a boot job to your Linode.
It's all the same to our system. Your domU was there, and then it wasn't. I don't think we know if you issued a 'reboot' from your Linode, because it probably tries to fire ACPI commands for a warm reset...which Xen might completely ignore.
These are all guesses, but educated ones. My suspicion is that 'reboot' is blindly saying "warm reset, please", Xen is ignoring it, and destroying the domain. This is probably why when you issue a reboot to your Linode from the Linode Manager, you get a "shutdown" job and a "boot" job.
(Aside: Higher host numbers aren't necessarily indicative of newer software. In this particular case, newark18 actually has "newer" software than newark35. We probably rebooted it at some point. Edit:
Yep.)