Well, not really a Fog Gate glitch at all. But more of something From Software prob didn't think through 100%.
In Undead Settlement (where I did this), if you haven't triggered the Boss fight yet, you can you know run into the boss room a little (quite a lot for that 1 Boss). You are free to enter and exit as you wish, but both co-op and invaders see a fog gate and are literally stuck. They can't even see you too (Not sure if it's 100% or just like see you a tiny bit).
So you can stand inside the boss room as you keep getting invaded and watch as confused invaders have no idea where you are! Heck, if an invader comes to the fog gate, you can still hurt them. I R2 my weapon to 1 of them and knocked them over and killed them with a 2nd hit after they got up. lol
The only issue I have found, is that I think sometimes it might auto-send the invaders back as soon as they invade if you are in there. So normally I stand just before the boss room and let them come (though not see me), and then run into boss room. Then they will try to search the map for you.