That’s because it’s treated as MultiMarkdown metadata. See Help > Documentation > Documents > Markdown Documents…
You need to add a Return at the top of the document. You can specify it in the Wiki template section, like so…
Secondly, anyone know of a method to automatically insert a back link into the new document (can’t see a placeholder for this) that would collect the item link from the document in which the wiki-link doc was created and place that into the document?
This is not possible at this time. DEVONthink would have to support the %documentLink% placeholder like you can use in Annotation files. @cgrunenberg ? 
Hmm. As an alternative…
Since you’re using square bracket syntax and Markdown, you can pass the name in square brackets, like so…
From this Wiki template…
It doesn’t create an item link (which I mentioned above), but there is a functional WikiLink in the new Wiki document.