I remember bottomless having this implication, but I just noticed it's gone for some reason. Which is extra strange because topless still appears to have it, AND suggesting the implication still returns the "Implication already exists" error.

What's the devil's going on here? =/

Still there.
Maybe you searched aliases by mistake. (I've certainly done that myself a couple of times...)

Then why didn't it get tagged along with bottomless when I uploaded two pictures with that tag? And the wiki doesn't mention it, either.

It's still pending.
Only topless is implicated. Bottomless was implicated long ago, but as far as I can remember, that implication got deleted almost as soon as it was approved. And it hasn't been restored yet.

I'm sure that I've had to tag it manually for... a few months, at least.

Just check forum #136810

Mmh, then I guess I must've been mistaken about half-dressed being implicated until recently, it just hasn't been at all.