Issue

// Works: Text is rendered as rich text
TextEdit {
    id: te1
    textFormat: TextEdit.RichText
    Component.onCompleted: te1.insert(te1.length, "<b>Hello</b> World")
}

// Does not work: Text is rendered as literal plaintext
TextEdit {
    id: te2
    textFormat: TextEdit.MarkdownText
    Component.onCompleted: te2.insert(te2.length, "# Hello\n\nWorld")
}

Workaround

Instead of calling insert(), use

te2.text = te2.text + "# Hello\n\nWorld"