Did you speak to Justinia again in Kemm's mansion? That might trigger a party dialog (I've gone through there with Beast both ways, but don't recall the details of whether he was an avatar or companion, or if there was a dialog after sparing Justinia).
That dialogue is presumably a mistake; it could be in character using 'human' broadly, if he didn't end the sentence saying "just as every lizard looks like every other lizard".
I did, and I also tried updating Micheil Ros about her, and there wasn't any dialogue (party or otherwise) after that, either. After posting this thread, I looked at a video of Beast's quest and romance by DanaDuchy on YouTube, and it definitely seems that there have always been exchanges missing from my runs. The video shows one in which Beast says "Things're gonna be alright..." etc. after sparing Justinia, and then another exchange when he's spoken to again that seems like a much cheerier version of the options that appear if you kill Justinia. But the video was not of DOS2: DE. In fact, I can't find any record of anyone playing the DE and getting these exchanges. I highly doubt this dialogue would be removed on purpose, especially since everyone else has things to say after their quests close, so maybe the upgrade to DE caused some problem?
Definitely a mistake, I think it's possible this happens with other dwarf characters as well. It's just amusing!