What security issues were found since we’re now being notified?

Updates tend to happen more frequently just after a major release. Once the new version is publicly released, it's not uncommon for problems to show up that didn't show up during the comparatively smaller beta release. That's normal. The two most recent releases for iOS 16 were not exclusively security releases.

Miscreants are always trying to find ways to compromise software. Software developers are always trying to find ways to stop them. It's something of an arms race. Many of the specifics are highly technical, well beyond the knowledge of most of us.

The bottom line is that you should always keep your device updated.

Security updates to software are routine. What is the issue?

Usually a possible exploit has been found which could cause a problem on the iPhone. No one gives specifics on these.