It turns out that the server is set to auto-suspend after 20 minutes of inactivity on the server console. Even with multiple active ssh sessions, the server goes into suspend mode if no one is logged in on the console. This leads to a couple of questions...
What is the reason that a new server installations defaults to enabling suspend mode after 20 minutes of inactivity? This is a server OS after all, not a desktop OS like Ubuntu where auto-suspend would make sense.
More importantly, why is "inactivity" only evaluated at the physical server console? How is it that remote network activity fails to keep it awake? Had I been a few seconds earlier with my mkfs.ext4 command, suspension would have occurred during the file system creation process!! A pretty picture this does not make.
The auto-suspend mode is easy enough to disable, but it really should not be needed:
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sudo systemctl mask sleep.target suspend.target hibernate.target hybrid-sleep.target
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sudo systemctl status sleep.target suspend.target hibernate.target hybrid-sleep.target