This may be an issue with Discourse, as opposed to the plugin.
If you’re using the default Wordpress emoji handling, I’m assuming you’re copy/pasting unicode emojis into the wordpress editor. If you do this the raw post content sent to discourse will just include the unicode emojis. Discourse should normally convert those into the emojiset of the forum. I can indeed reproduce the issue in certain circumstances.
I’ll have to get back to you on this one.