My current char, several saves and levels ago was going through the areas around south vegas ruins, Cook-Cooks general area. At that time
I was getting good karma for the wholesale slaugther of Fiends. Went back to that area recently and noticed I wasn't getting any karma points, So, I loaded the old
save I had when I was going through there the first time. Still not gaining any karma from killing them despite them being marked as evil.

I'm not using any mods that change karma acquisition, in fact, I unloaded all my mods just to be sure something wasn't interfering, but no dice.

Did the recent patch change something that would affect that?

I don't know this for fact, but I think either clvl or karma level or something to that effect seems to increase the number of fiend kills needed to gain that karma message. I could be wrong tho...it's just felt like that. I know over time if I keep going back there, I still get the occasional karma for killing the random/respawning ones, but the periods between becomes longer and longer. Decreasing returns, so to speak.

You only gain Kama if you sneak kill them

Hm nope that seems wrong. But yes I doesnt always get karma killing bad guys. Perhaps its random for the average mister Bad, in case of Powder Gangers I say 1 in 3 ?!
Oh and npcs and possibly companions seems to steal Karma if they score the final hit, (I think).