Actually, I believe it only happens for low rank missions. For higher rank around threat level 8 I’ve been getting ambushed. 1 Anaconda as you tag him his buddies come to meet you.I used to enjoy the chaos of jumping into a group of pirates in one of the threat 3/4 signals and just going to town as all of them opened up on my Corvette - great way to put those shields to the test.
Then someone at Frontier flicked a switch, whether on purpose or accidentally. so they ignore you blasting their buddies into the void. It also ruins that scenario with a lone Anaconda waiting and a supposed ‘ambush’ of pirates… that scan you and off when you attack the big ship.
I focus my drone on the Conda and solo the little guys.