I can only guess because I've been a redhat/fedora person since 1998(?) but likely an Upstream or Imported package isn't signed. Or possibly the distro maintainers at mint don't do a good job and some of the main packages in their repository are not signed ( which would be a deal-breaker for me, honestly ).

If you can track down the root of the problem it would be a win. Signed packages are a big deal to me. IMHO a distro that provides a working/usable/requires-no-headaches SELinux base config is also a big deal ( because when SELinux breaks things most people just disable it -- which i totally get ).

does it log which particular packages were unsigned? you might be able to see origin in package metainfo....