Basically, in the 'world' the FB no longer exists, but Infantry are still able to spawn on it because the spawn point is available on spawn screen.
I have experienced this from both teams point of view. The most prominent one, taking place on Muttrah (iirc it's common on muttrah but iv'e experienced on other maps also).
So I was on US Team - we had taken north city but were confused to find Enemy Inf still behind our lines. We were certain everything behind us had been cleared out. After tracking where they kept coming from, it turned out that MEC had been spawning at a location where one of their FBs used to exist, which had been destroyed completely off the map earlier on.
I was SL and I told my guys the situation, and that we had to keep it under control because if we didn't, we'd get overun - something which did eventually happen, but managed to regain after some more fighting.
We had to circle the invisble FB and litterally spawn kill anyone who spawned. There was no alternative. Below you can see I deployed some assets to reduce spawn 'surface area' to makes sure none evaded us.
I did notice the well known "spawn dot" above the wall, which you can slightly see here (this white dot is known to appear when assets such as Fobs are built (look in known bugs section - under deployable assets).
The enemy were still able to click the spawn location on their spawn map after that relevant FB had been destroyed. I know this because Ive also been in the situation where I have found myself, able to select a spawn location on muttrah, where no FB actually existed (no green/yellow triangle on the map) and actually spawn there as normal.
