If you type (see #guides) it should work (link the project hashtag). It doesn't, currently:
(see Guides)
Likely the ) is getting caught by the regexp. This works fine:
(see the Guides tag)
I am unable to un-associate Guides to this task, also. haha.
Hmm, I think it auto-reassociates because it's in the description.
For posterity, T6819 discusses the Guides issue specifically.