I think this is not a Xenforo bug just a limitation of how bbcode works, if there isn't a closing tag it will make one
For it to work you have to have a closing bbcode when you post or else it gets added after the text.
This will work.
Code:
Text here
[hr][/hr]
More text here
Obviously users probably won't know that it therefore makes most sense if you have a text editor icon for the bbcode which will automatically enter opening and closing tags,
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Differences Between HTML 4.01 and HTML5
In HTML5, the <hr> tag defines a thematic break.
In HTML 4.01, the <hr> tag represents a horizontal rule.
However, the <hr> tag may still be displayed as a horizontal rule in visual browsers, but is now defined in semantic terms, rather than presentational terms.
All the layout attributes are removed in HTML5. Use CSS instead.
One annoying thing about this is that if you invoke the bbcode via an icon in the text editor, the cursor will be there blinking between the [hr] and the [/hr] so tempting your users to put the next text there as opposed to after the [/hr]
I doubt this is a xenforo issue though. In fact I found the solution above from a phpbb support posts. (Sorry again about missing the advanced html parameter!)