As long as the more valuable planets retain some kind of mutual defense pact, it's probably not worth the effort to attack the coalition: too little gain for the effort involved. That doesn't require an active central government, although the AC could very well still exist in name, but with no real authority or outside recognition beyond the couple of planets at the very core. Whether Kamea or her successor(s) still govern isn't all that relevant.
If the AC is a going concern, with a sizable military force and control over a dozen or more planets, then their lack of mention on the maps becomes a lot less plausible, and Comstar certainly can't hide their presence from the Magistracy and Taurians. Fronc would know about them, but might not have any direct concerns if they're not in some kind of dispute over any planets, particularly if they're nearby but not actually bordering them.