Unable to get original content for LinkReferenceDefinitionGroup

I'm parsing a Markdown document that contains LinkReferenceDefinitionGroup . When I call ToPositionText() method for that part of the MarkdownDocument, the line and column will be 0 and 0, and the Range is wrong too. I don't know how to fix this. Any hint?

var document = Markdown.Parse(markdownText);
var parts = document.ToList();
for (int i = 0; i < parts.Count; i++)
{
    var item = parts[i];
    if (item is Markdig.Syntax.LinkReferenceDefinitionGroup)
    {
        // item.Line = 0
        // item.Column = 0
        // item.ToPositionText(); // $0, 0, 0-79
    }
}

Start by reproducing the sample with the Markdown.Parse call and inspect LinkReferenceDefinitionGroup, its Line and Column values, and ToPositionText(). Trace how its source range is assigned compared with other parsed nodes; done means the group reports the correct position text, line, column, and range for the referenced Markdown.