Ok, please try v1.1.2 of the Goodreads plugin from
here.
The issue only arose on Calibre 0.7.48 for books that have no tags coming through from Goodreads, but do have some from Google Books. My plugin was returning "None" in this situation, now it will return an empty list. Perhaps my plugin should always have done this but with hundreds of downloads I feel sure someone else would have noticed the issue before now.
I still don't know *exactly* why that leads to the Google Books tags being cached on a Metadata object for any new book that gets added. However I believe it is something to do with the changes that have been made recently - whether Kovid deems it worthy of investigation is up to him as we have a workaround.
Let me know how you get on with it.