Yeah, it’s auto-edited out after you post it. ^^^
See if it works this time…
It worked that time because the post you were quoting wasn’t the very last one.
And again. Maybe I have figured out the issue…
Nope. It seems to happen randomly. I guess it’s a bug.
When you mentioned ‘last post’ before, I didn’t realize that you meant the last post in the thread. I thought you meant the last quoted post among quotes…if that makes sense.
I’ll try it here now…
Testing…
Ta da! 
Once more, just to be sure…
There must be some hidden formatting character getting deleted?
I was able to quote the last post of the thread if my text was placed above the quote.
Testing with quote before this text and after this text.
It’s fuckery, I tell you…
@Snookoda Yep, there is some hidden character.
Try this: Quote the last post and watch the preview at the right. Hit backspace until you get to the last square bracket (not removing it). You should see the quote disappear in the preview.
Now I can’t reproduce the preview thing. Fuckery…
But the last quoted post did disappear from this post.
Odd that that happens. You can see the quote disappear as you submit your post.
.
A stupid workaround. Adding a character before quoting the last post of the thread.
Trying again by adding a space just before quoting the last post of the thread…
It works. But yes, it’s fuckery.
I can’t reproduce the preview thing here either. The logic appears to be:
If the start of the post is a full quote of the post directing preceding it, auto-edit it out.
Adding a character is as good a workaround as has ever existed! 
I think any character should work. In this post I moved my cursor just before the quote and hit enter, then entered my reply below that.
…Nope. More fuckery. Ha ha. But yea, space works.
I found the offending logic in the code:
… where you can see that there is apparently a site settings to remove this abomination!