There's an inexplicable bug in some of the tags which causes the dropdown to link to an invalid destination. I first noticed that, for example, the procedural-generation (ID 264) tag on Game Development does not show an excerpt, despite it having one. Further investigation showed that the "help us edit this wiki" link goes to
https://gamedev.stackexchange.com/tags/edit-tag-wiki/264, which is not a valid destination (the correct one would be without "tags/", so
https://gamedev.stackexchange.com/edit-tag-wiki/264). For now, I'm assuming that this is all one bug.
Upon further investigation, there are various empty tags which exhibit this same linking to the wrong destination. It's not every empty tag, and it's not just on Game Development. On Game Development, it affects "2d" (ID 24) and "3d" (ID 11), but not "engine" (ID 25) or iphone (ID 3). You can see it on this site with badge-request, which apparently does work if I used the right syntax, you can observe this behavior.
It seems very, very haphazard in which tags it affects and which it does not affect.