Linked graphics problem -- relative paths becoming absolute?

I'm using FM6. My graphics sub-folder is in the same folder as my document. From what the Help says, FM should be using relative paths for the graphics. And indeed, it does most of the time, because I'm usually able to copy the whole folder with the document and the graphics subfolder to another location, and open the document with no problem.

However, sometimes when I move the folder and then open the document, I have to tell FM where all the graphics files are again, even though their relative position hasn't changed. This is a real pain because there are two sub-folders in the graphics folder, and I have to keep pointing to the correct sub-folder for each file.

Does anyone know why this would happen? Is it a known issue in FM6, and is it something I can fix? Unfortunately, the company has no budget for stuff like this, so any solutions have to be free. Also, the graphics folder with the two subfolders is a customer requirement. Even if putting all the graphics together in one folder would help, I can't do that.

Thanks for any suggestions.