Ultimate F*ckYou, er Ultimate Frontier is the name of the last mission.
Also what is worse for these 3 extra BBs that spawn for Aegis, is that they are worth _0_ XP and Credits.
Back during the glory days of Operations, I was part of a Discord built just for Operations. And we tried to squeeze every xp and credit from each Operation. We tried on purposely spawning those 3 extra BBs, and no improvements were seen at the end. Not worth spawning them at all.
Also note: This 'feature' has been there since the start, 4+ years ago, in Aegis. Just usually no one rushes ahead, so they aren't triggered. Or as someone else mentioned, they are triggered if it takes way too long to kill the 5 guards on the 5 cargo ships.