If you're not on mobile, you can click on the reply button beneath the post, you wish to quote, and then click the button in the top corner of the post window that looks like a speech bubble. That will quote the whole post (coding and formatting included). Then, if you wish, you can trim it down to only the part you want to quote.
(Also, this is a good way to figure out how to do things you don't know how to do -- find someone who has and see what they typed)
If those three methods don't work, I'd recommend asking a mod. Probably a bug.