I mean, it always felt that way, even though they claimed all the BP rates were equal. People reporting stuff like 20+ Stalker kills with no BPs, getting 7-8 Dreads and no Hate or Despair, all that fun stuff. Though apparently that's still the case, there are people with data less than a year old showing ~33% BP drop rate, and <10% drop rate for non-Dread BPs. Was mostly just happy because I've already got my Paris Prime riding the Forma train and have a lot more Red Veil grinding to go before Rakta Cernos. Don't really want any of the others as much.
I've had the Stalker pop up a couple times, despite the fact that I have hardly farmed any bosses... Lech Krill for Frost and PHORID for Fieldrons, yet I've seen him twice.
The first time he wrecked me because I wasn't careful, and the second time he popped up while some Corpus mall cops were beating me to death, gave a look of approval and left when they finished.
Yeah, that's been my experience about half the time, getting blindsided because I didn't see where they spawned in. The other half, of course, is them showing up right in the middle of a four-man farm and getting killed before they even stand up.
It's really, really easy if you're running a CC-heavy 'frame like Frost, completely trivializes some of them (Stalker in particular).