Not sure if it's a Pixel 6/Pro or bigger bug, but I have Chat on in Messages and can reply with emoji when I long press a few other users with the feature. I can usually long press others to copy or delete a specific text with any user in any thread, but when I tried this morning it wouldn't let me select a non-Chat user's text at all, and then when I went to a Chat message it wouldn't work there either until I backed out of the app.