If this is separate and unreleated, I'll post it to a new thread, but it sounds very familiar - except I don't auto clean my download or par2 queues.
I have seen something similar - the par2 check realises it needs to download an additional par2 file to repair the set, and it usually grabs a file based on required blocks to repair, but then for whatever reason the par2 repair just doesn't complete. It goes through the process, but no repair.
This could be because there still aren't enough blocks or it "gets confused" and just wont repair for some unknown reason.
To get around this, in the past I've manually unpaused an additional par2 file from the download queue, once that finishes downloading, this prompts the par2 check to check over the entire fileset again, and often that is enough to continue to the process of repair and unrar and subsequent cleanup.
But on a semi regular basis, it just wont complete - I end up using Quickpar or Multipar to repair and then I can right-click the par2 set and select the "unrar par2/sfv" (CTRL-U) option which then unrars and cleans up the files for me.
If you need extracts of logs, I can probably find something, or just wait until it happens again.
Thanks.