It's done and didn't change anything, but I think I finally know why : running apt list --upgradable reveals the one outdated package is xdg-desktop-portal, which I used apt-mark hold on because of an issue preventing screen capture/sharing from working that affects even the latest version.
Sorry, I don't really have an easy answer for you. First, don't hold packages, of course that always leads to problems. Especially for release upgrades. Then try to get fully up to date, and if the release upgrade doesn't work after that it might make more sense to reinstall. In that case backup your /home, reinstall the system, and restore your home.
To preface, I am a Debian user, not a Neon user, but I imagine this will still work on Neon given it's based on Ubuntu, but I too have had Discover mess up updates and I've found that simply trying to update again with nala just works.