I attempted to replicate this, was a dwarf grabbed two drunks, killed one in the woods, my companion reacted negatively too it, and then i fast traveled, and they pretended everything was fine and so this bug exists, (I have seen a bug where drunks get duplicated, so if i go back zon may be duplicated and that would explain the issue) I still think your guess as to what the issue is is wrong, as if i kill a big bad my companions will know usually. (A roc or some such) but eh, ima do some more tests.
Told them again, they pretended it was fine after a fast travel (still the dwarf kill)
Killed giantess
, companion thought i was "very brave"
Later on after fast travel i asked about the cave i killed the giantess in and my companion talked about how i killed it. But when i ask about myself they still dont mention it. So they must know at some level.
But when i ask about the giantess directly, they say they are a dangerous animal and dont say anything about them being dead.
Someone should report this on the bug tracker, its easy to replicate. But i cant ATM as its late and im going to bed
I reported it, please fill this bug report with comments containing your reports and im sure he will fix it.
https://www.bay12games.com/dwarves/mantisbt/view.php?id=11217