(since for some reason I'm unable to reply), the reason you don't get any upgrades listed when you run lsb_release -a on 18.04 is because 18.04 is an LTS release, so by default it will only look for the next LTS version.
I think, though I'm not 100% sure, but I think you can change this behavior by editing /etc/update-manager/release-upgrades. Change the line Prompt=lts to Prompt=normal and I think it should work. I could be wrong, though; I've never actually done it because I generally stick to the LTS versions.
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