It's based on the old "likes" implemented first, not the new emoji "likes". But the old "like" icon is only seen under certain conditions such as a when the post is viewed as a reply to a previous post. It's confusing. I mentioned this a month ago when this subject also came up.
Is the "Popular" function broken on all boards or just a few ? The ones I have checked show only 1 or 2 thumbs up on them.