Confirmed this cosmetic bug exists in both 6.51 B3 & B5 b3154. With two RAR sets in the download list, the top one is collapsed (showing only 1 line) and the bottom one is expanded (showing each RAR/PAR). If I attempt to move the top download 1 line down, it splits the expanded list. Now the split list (which is now on top) can be collapsed and expanded, but the files that were split on the bottom appear to be orphaned.
This doesn't affect the download. The separate files from the split download can simply appear twice in the list. Once the RAR set is downloaded, the orphans end up blank with a red bang(!). Using x64 version on Windows 7 x64 Pro SP1.