There are definitely differences in facial expressions between races and the Durge will occasionally make a slight, what I call Durge face. It's not quite as pronounced as the moments when the screen turns red, but there's clearly a malicious greed to be seen.
I once watched videos of this scene from my previous playthroughs. It depends on which dialog choice the Durge makes. If I just say to Wyll that I want to talk to him, then a scene occurs in which our tadpoles connect. If I tell Wyll that this whelp is a hopeless case, then I look at the child with a slightly greeded evil face. The extreme grimace like in the picture above seems to come about when the character smiles at the same time, which the character does random in conversations.
But yes, I am revising, the expression is not new, it existed before the patch and without a smile it looks much milder.
This is from a run before Patch7:
Durge face before screen turns red:
Again the extreme expression when it comes together with a smile... you get a little scared
Somehow, with the Tiefling children, the Durge seems to react with these faces to certain dialog choices or events.
Take a closer look at your Durges' facial expressions in certain situations, I'd like to see what it looks like on other races.