I checked into this one and it is a known issue that has already been sent along to be worked on 
In the future if you notice a bug like this then submitting it via the in-game bug report option is the preferred route. To report a bug inside of WoW, press Esc to open the menu, select the Support button and on the page that opens, below the search box, the second button in says “Submit feedback or bug report”, use this.
If you cannot see the button then this post may help.