The icon works if you can’t like it anymore, but not when you can.
Looking at the code, when you liked already:
When you did not like yet:
It expects a far— version, which doesn’t seem to exist for beer. 
A quick solution in your case is to use the API to replace the far-beer, for example:
<script type="text/discourse-plugin" version="0.8">
api.replaceIcon("far-beer", "beer");
</script>
I don’t know if a new setting should be introduced here or add some logic to use the same non-far version if the far-version doesn’t exist.