As damionx7/Zettel-Notes-Documentation#12 (comment) (and damionx7/Zettel-Notes-Documentation#12 (comment)) explain:
works, whereas
does not. However, this is not limited to unfilled lists – checkboxes do not appear to render anywhere outside of syntactically perfect lists. I believe that I should be able to place a checkbox anywhere, and can with most other software.