Using 6.52b3 bd3196 I see occasional UnR*******_tmp folders.
I have over 1TB of free space on the disk & NB doesn't do it on all downloads.
I also see the odd .nb2 file.
It's not all the time & 99% of the time everything seems to be fine - things have repaired &/or extracted, files aren't corrupt. <1% may need manual extraction.
I don't see a specific pattern - size, number of files, age etc seem irrelevant. But a weird 'partial' cleanup completion state occurs sometimes.
It's not a big deal for me but figured you may want to know about it.