When “Only groups” is enabled in Settings > WikiLinks, clicking on a new square-bracketed wikilink (e.g. [[ABCDEF]] ) will create a new document named ABCDEF, not a new group. Yet the wikilink is inhibited from pointing to the new document. Therefore, every time I click on the bracketed wikilink in the editing pane, DT generates a new document without wiring it back to the source document.
I think it would be more consistent under “Only groups” if the wikilink creates a new group under the grandparent of the source document. For example, if I have the structure Group A > Group B > Document C, and a new wikilink [[Group D]] in Document C, then clicking on the wikilink should create Group D as a sibling of Group B.