Special:MobileCite should not have a rel canoncial link, nor should it be indexable for search engines. Visit https://en.m.wikipedia.org/wiki/Special:MobileCite/722891576?mobileaction=beta for an example.
Presently you'll see something like the following in the <head>:
<link rel="canonical" href="https://en.wikipedia.org/wiki/Special:MobileCite/722891576"/>
But we wouldn't want to point at the desktop domain for this page.
And you'll notice the following is absent:
<meta name="robots" content="noindex,nofollow"/>
Although the Description in T136617 suggests that the non-indexing directive is in place it in fact is missing. The discussion on that task suggests we really don't want these pages indexed, so we should make sure the directive is in place.