I’m not even sure sarcastic is the right word to describe it. It’s usually when someone is saying something nice but it isn’t meant to be taken nicely. It isn’t the word “snarky”, and I think it’s more referring to the entire phrase/word rather than the person saying it. I’m imagining it happens when someone does something nice for someone and the other person comments almost-sarcastically, like “Yeah, that’ll help.” or no no, it’s more like when you’re asking someone to do something but you’re kinda beating around the bush, kinda snarky. For example, you’re making a comment about how someone should do the dishes without saying straightforward, and maybe there’s a bit of disdain for that person. Say, the person who should do the dishes is right there and the other person says “Man, the dishes are really piling up,” with a kind of nice tone.