I think they're synonomous, but the key part to me is that "lying" is directly related to "liar", whereas "untruthful" doesn't have that relation with another w

I think they're synonomous, but the key part to me is that "lying" is directly related to "liar", whereas "untruthful" doesn't have that relation with another word, at least to my knowledge. So saying that someone is "lying" is also attacking their character by implying that they are a "liar", while "untruthful" could mean that even though they are saying something that isn't true, it doesn't necessarily reflect on their character as a whole. It's just playing semantics, but I think it is treated different on a subconsious level.