Hmmm what a strange bug..,, looks pretty cool though.
Looks like a custom map, does this only happen in this map/the editor or also in the main campaign(s)?
Have you added or changed anything in the items folder? specifically the item.png in your custom map folder?
Below you will find my version of the valuable_9.xml and items.png so you can compare them with your files.
The valuable_9.xml links to a specific area in the items.png for the sprite it uses, Maybe your code in the xml or location of the sprite in the items.png has changed somehow?
My version of the files as found in the Hammerwatch folder: http://we.tl/nDAtGN2XcnHope the issue is resolved soon