Bug overview description.
Tags with a negative count, or at least one such tag, doesn't show up on tag and wiki search engines, but work fine on the post search engine (also a couple of minor site errors)

What part(s) of the site page(s) are affected?
The search engines at the 'Tags' page and the 'Wiki' page.

What is the expected behavior?
Search should return all results, even ones with negative post counts.

What actual behavior is given instead?
Tags with negative post count, or at least this particular tag with negative post count, don't show up.

Time of incident (if applicable).
n/a

Can you reproduce the bug every time?
Yes.

What steps did you take to replicate this bug?
Just use the pages' search engines:
/tag?name=artist_signature
/wiki?title=artist_signature
The tag's own wiki page doesn't display the posts: artist_signature
Notice how post search works fine tho: artist_signature

Errors or other messages returned (if any).
n/a

------------

While I'm at it, a couple of gripes. One, the first 3 examples in the "Links" section at the Help:DText page seem to be borked due to escaping shenanigans. Two, my browser console won't stop bothering me about this, to various revisions of the file at the same time: [JavaScript Warning: "Use of getPreventDefault() is deprecated. Use defaultPrevented instead." {file: "\https://e621.net/javascripts/application-min.js?1531978946" line: 473}

By default the tags page doesn't display this tag because by default it doesn't return results for empty tags, which are defined as anything that has 0 or fewer posts associated with it. There is no wiki page with that title, so it won't show up on the wiki. And the wiki has an optimization where it doesn't show posts if the cached count is zero or lower as an optimization.

This specific tag is actually aliased to signature, which is why it doesn't appear to behave correctly, since these specific pages don't play attention or resolve tag aliases before determining post count, but the search system does.