Is it visually countering them, or are they just not shooting? That sounds like either Missile Turrets count as dark matter but don't display as being such, or that AI (core) FFs are immune to turret shots somehow (which would make them not shoot at anything under them). That's pretty odd really, either way - the tooltips don't support either case being true. I guess FFs being immune to turret shots specifically might make sense, but honestly if you have a planet in supply and are willing to piss it off by building turrets on them and want to wait hours for them to kill a FF that doesn't sound especially problematic. Same with your situation and being willing to perma-alert a core AI planet to put it in supply and build turrets on it just to keep FFs from regenning, especially since that's pretty much guaranteed to be pissing off at least one other core world, probably including the homeworld.
