This does seem to be the case. Reviewing the "recently created tag wikis" that 10k and diamonds can see, newly created wikis come in the following order around the time of yours.
- identify-this-game (From Oak, has wiki and excerpt, has tooltip)
- local-multiplayer (From Oak, has only excerpt, no tooltip)
- fallout-new-vegas (From Fabian aka you, has wiki and excerpt, has tooltip)
- sandbox (From badp, has only excerpt, no tooltip)
- dragonage-origins (From Fabian, has only excerpt, no tooltip)
- ps3 (From Oak, has only excerpt, no tooltip)
I could go further, but it already demonstrates that many of these are already past the normal cache date. The only consistent factor is the presence or abscence of the wiki. So, indeed, there does seem to be a bug that occurs if you enter an excerpt but leave the wiki blank.
According to the list, everyone is actually credited with creating the wiki in addition to the excerpt. So it seems to actually detect whether there is textual content in the tag wiki, rather than if the wiki exists. And on detection of nothing but blankness, it goes with the "help us create a wiki" message.
Makes me wonder if that means that this earlier request is effectively implemented now...