Under MW 1.26.3 and 1.27.
When canonical URL follow an ultra-short http[s]://(mydomain)/Page_title scheme and a page with the title beginning with a slash (/) is requested, an endless permanent redirect (301) is served. It happens for both http[s]://(mydomain)//etc and http[s]://(mydomain)/w/index.php?title=/etc-type requests for the page titled "/etc".
The metod WebRequest::getGlobalRequestURL() (lines 792-799) "normalises" the URL by removing all leading slashes but one, and then MediaWiki::tryNormaliseRedirect () in lines 344-348 redirects.
Although, in my opinion, the error is in WebRequest::getGlobalRequestURL(), and was there for some time; it was revealed by introducing MediaWiki class in MW 1.26.3.
There is a similar, though less severe, bug T113160. But this one blocks upgrading to MW 1.26.3 or MW 1.27.