Summary pretty much says it all- thank you to Norberto Bensa for questioning this on the gentoo-dev mailing list. There is a subtle logic error in fetch that only raises it's head when a DISTDIR is readonly, and a file is fully fetched. I'll be attaching a patch shortly.
Created attachment 39035 [details, diff] fetch-fix.patch Corrects the logic issue.
Bug has been fixed and released in stable portages on or before 2.0.51-r2