After download is complete, the downloaded files are named as:
After renaming manually, the files are fine.
As far as I can tell, this happens with larger files only, for example when a single file is about 700 MB (about 550 sections/segments/parts(whatever you call them)).
Smaller files, such as rar parts, are renamed correctly.
I don't have MemCacheLimit set to anything, possibly setting it to a larger value would help, not tested.
But I think it is a bug anyway.
No big deal, because when files are so large, their number is usually quite small, therefore easy to rename manually.