My partner's animosity towards our friend is stressing me out.

My partner (33F) has developed a strong dislike for our neighbor (20sF), which has put a strain on our social circle. We used to hang out regularly with other neighbors, watching TV and playing games, but now my partner can hardly stand her. The way she speaks about our neighbor is often vulgar, and it makes me uncomfortable. She's made hurtful comments suggesting that I like the neighbor, which I know she doesn’t really believe, but it still stings. My partner often refers to the neighbor as "my friend" in a sarcastic manner, even though our interactions have always been casual. This sudden animosity has affected our entire group dynamic. Just yesterday, the neighbor and her partner (also in their 20s) stopped me while I was out walking to express their concern about my partner, asking if they could help in any way. I can't be honest with them about the tension, though. My partner criticizes the neighbor for trivial things, even while doing the same herself, which feels hypocritical to me. She thinks the neighbor is selfish, and when I chatted with the neighbor outside recently, my partner had her headphones in to avoid hearing our conversation. This negativity is really unappealing to me, and whenever I bring it up, my partner becomes defensive, insisting that she doesn't have to like her or her values. However, her unwillingness to even pretend is impacting my relationships with both of them. I'm at a loss for what to do or why this has all unfolded, and I genuinely don't believe there's any underlying attraction in play.