This was modified to show only the tag synonyms which you are capable of voting on. The limit is based on your upvotes in the tags, although the restriction was recently lightened and so I don't know the exact number. waffles goes into details on the original changes in this post.
I changed the "suggested" filter on the UI to filter out all suggestions you are unable to vote on. (note: it will still show ones you have voted on, but the list should be pretty small - so I do not anticipate this being a problem)
In essence, if you can't see a synonym, you can't vote on it. Unless you're a diamond, but I've already put a request in for that.