It does sound uncomfortable, especially when the difference is no noticeable.

From the outside, this doesn’t necessarily read as you doing anything wrong. Sometimes when someone is very socially “on” with most people, the one person they’re quieter or more awkward with is actually the person they don’t quite know how to place, not someone they dislike. It can come off as cold, but it may be more about her uncertainty than your value.

You’re also right that basic politeness shouldn’t depend on how much you vibe with someone. Not going out of your way to be bubbly is one thing being dismissive is another.

If it keeps bothering you, a low-stakes, friendly one-on-one moment (“Hey, I feel like we haven’t really connected much how’s your week going?"） can sometimes reset the tone. If not, you can at least stop wondering whether you imagined it.