IMO it's not a bug if you include "wrong HTML ".Description: You may use HTML
The text (or HTML) you insert here must be valid within a <p> tag.
Edit: I see you mean forum description, not board description. Specifically this forum: http://demo.xenforo.com/104/index.php?forums/test.3/Enter a description for your board. This will be placed inside the meta description tag for the forum home page, so avoid using HTML.