I did some testing on this exact thing, and it seems that if your ship has mining fighters, it checks for fighters "on the ship" to see what it can mine. So even if you high enough tiered fighters, if they're deployed and out of the ship, when checked it doesn't detect a fighter high enough that's in the ship.
I also suspect that this then sends a variable stating that it can't mine anything, and completely skips even checking for turrets...but that's just a guess based on behavior I've seen so far.
