I had this same problem on a previous release, but I can't remember which one. Perhaps it may be because when I download, I right click and choose "download to existing folder" in order to keep the downloads organized in the download folder?
I try to download a file with multiple .rars, and it only downloads a portion of them. I have to then open the .par in Quickpar, note the files that are missing, go back to Newsbin, click on the "+" to expand the list of files, and download each of the missing ones individually. I didn't have the problem in 6.80B4, nor--IIRC--in the last couple before that. But as I said, it happened a while back with a previous release.
Once I've downloaded all the missing files, I can repair (if needed) and unrar the file, but I have to do it manually; Newsbin will no longer do either automatically. It seems as though it's happened with every multi-rar file I've downloaded since the B5 update, but I'm not 100 percent sure of that.
One other thing that I noticed: if I expand the main header in the download list, the .rar files are listed out of order, even though they're in the correct order when I expand them in the header list window.
UPDATE: Installed build 4909, and the problem seems to have gone away.