Shouldn't there be a badge for creating a tag wiki entry?
And should it be retroactive?
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That behavior seems kind of weird to me too, but I'm not sure it's worth "fixing." As time goes by, more and more tags will already have wikis, so it will be harder and harder for anyone to actually get new "created a tag wiki" badges (though, admittedly, Taxonomist suffers from the same issue).
Also, anyone who's planning on creating a wiki could always just make a small edit
immediately a little more than five minutes after posting the initial version to get the badge (thanks for pointing that detail out, @ChrisF!). So if you care that much, the badge is open to you.