Per Old user avatars in quoted text? - #5 by gerhard, I believe changing the avatar should schedule a rebake… this may take some time and won’t be instant.
If you’re an admin, you can try to manually use rebuild HTML from this menu within under the post to see if that updates the avatar:
If that works… then rebaking is working as expected, but the automatic process either hasn’t made it to all the posts yet, or has failed for some reason.