Pdf format doesn't handle a lot of transparency types. In fact, handling transparency at all in pdf format is a relatively recent development. So, the issue may be that you're using a version of the pdf format that doesn't know what to do with transparency. Or, since you have a fairly complex image, it may be that the pdf filter is having trouble figuring out what to do with all the elements within its limitations.

The bitmap route is the best way — and sometimes the only way — to eliminate the ghost.