Yes, one actively exploited CVE was patched in iOS 12, which is strange. But as your own link shows, that's the only thing that has been fixed in pre-15 versions since October, despite many many vulnerabilities patched since then in iOS 15.

I don't know why Apple patched that 1 bug in iOS 12. But they didn't patch it in iOS 14 or 13. Moreover, iOS 14 hasn't received any security updates since October, and other than that 1 bug, neither iOS 13 nor iOS 12 have received security updates in an even longer time.

You're treating the exception (12.5.6) as if it's the rule, but it's not.