Hmm, it shouldn't. The docs say: "You may exclude URIs by defining their routes outside of the web middleware group that is included in the default routes.php file, or by adding the URIs to the $except property of the VerifyCsrfToken middleware"
Try moving that route outside of the group as that will definitely exclude it from VerfyCsrfToken.
I'm on 5.2 (upgraded from 5.1) so I still have the 5.1 Kernel thus all my routes are subject to the global middleware that's been moved to 'web'. I have the except property set just as you do for my Mandrill webhook which goes through the Csrf middleware without issue.