Identified the problem.

The issue is plain text is eating the line break so paragraph is failing since it expects a line break at the end.

src/parser/document.rs

@@ -83,8 +83,7 @@ pub fn document(input: &str) -> Res<&str, Document> {
     let section_matcher = parser_with_context!(section)(&document_context);
     let heading_matcher = parser_with_context!(heading)(&document_context);
     let (remaining, zeroth_section) = opt(section_matcher)(input)?;
-    // let (remaining, children) = many0(heading_matcher)(remaining)?;
-    let children = Vec::new();
+    let (remaining, children) = many0(heading_matcher)(remaining)?;
     let source = get_consumed(input, remaining);
     Ok((
         remaining,