Rapid Start was already disabled on my machine so it's one of Smart Connect or TPM that did it. With all three of those disabled, the system does not resume after 24 hours.
I'd still argue this is a bug, though, as all of these should function under Ubuntu, no?
Issue resolved. The magic seems to be:
Disable Rapid Start
Disable Smart Connect
Disable TPM
Rapid Start was already disabled on my machine so it's one of Smart Connect or TPM that did it. With all three of those disabled, the system does not resume after 24 hours.
I'd still argue this is a bug, though, as all of these should function under Ubuntu, no?