When you try to link to imgur, even if you explicitly tell it that it is a URL in bb code, it converts it to media...
Try this and remove the spaces: https:// imgur.com/upload/
You get this:
That's not really a bug, but more of a feature that has an inconvenient side effect when you're linking to some random imgur pages instead of linking directly to a gallery.
I think that maybe Tony could add some exceptions to the function of xenforo that does this, but I'm not sure how easy that is to do... And well, he doesn't really develop the forums, so it's unlikely to see this kind of fix for a small inconvenience...
That said, you don't need to use spaces in the middle of the URL to break it. You can just use the
[CODE][/CODE] or the
[PLAIN][/PLAIN] functions to make the URL be looked at as if it was plain text.