In DSpace 5.8 and prior 5.x, when no primary bitstream was set, the item list view (such as search, browse) will show thumbnail of the first bitstream. However, in DSpace 5.9 and later, the thumbnail of the last bitstream is shown instead.
Also refer to this email thread: http://dspace.2283337.n4.nabble.com/5-10-Discover-Search-showing-final-thumbnail-not-first-td4689566.html
The change was caused by performance improvement in
DS-3883 which limit thumbnail information send to Cocoon to just only one (renderFileSection() in objectmanager/ItemAdapter.java). But somehow, the thumbnail included is the last one instead of the intended first one.