Hi @adhi_databricks.,
Good day!

Please check for the probable errors:

• It is a Code cell, not markdown - Change to markdown cell, then add heading
• Incorrect heading syntax - Try using # Heading or #Heading and then refresh the page
• No headings/titles - Add a markdown heading or cell title
• UI rendering issue - Clear cache, Refresh browser and re-run notebook.

If none of these help, please consider raising a support ticket with screenshots, as this may indicate a UI bug.