For that test I've used a rebuilt archive, not the one you had in your archive. The problem is based on an archive file index / order problem.
When a rename operation fails (and in that case, 2 renames fail since the right name already exists in the archive), it moves the not renamed ones to backup (so they can be picked up later on again). So it made a backup of file 5 and 6 for example. So far so good. Unfortunately, it directly removed the files, so it copied 5 to backup, removed 5, copied 6, removed 6 without having the indexes refreshed in between. So when it removed 5, copying 6 was simply the wrong file.
In my case the archive order was simply the other way around. It copied 6, removed 6, copied 5, removed 5, so it always picked the correct ones. To understand this: The backup copy is index based, the removal is filename based at that point...
So it only happens when more than 1 file failed during rename (where the right name files also exists in the archive)
The fix is pretty simple, do the removal of all of the backup'ed files after the copy....