It's possible to lose external images files from an office preview when several previews of the same attachment are done with different parameters

Whatever the parameters used the cached office viewer transformation share the same external files and when one is removed from the cache it remove those images. The only way for it to happen only for one of the cached XDOM is if the cache is full.

Note that when using a recent LibreOffice (like 4.2+) the image are usually directly included in the HTML so that issue cannot really happen since there is no separated images files anymore.