This is noted in the release notes, and I have a blog post on it here: http://www.tocker.ca/2015/03/10/heads-up-mysql-5-7-dmr6-cont...

To describe our process: We have at least 2 code reviews, ~90% test coverage before we release a feature in a development milestone. It was then caught by at least 3 people that I know of prior to public release. Since this is not a GA release, it was decided to fix the bug in the next version.