DXLExporter bug ? alttext attribute for pictures using DXLExporter

I am trying to create a DXL file for a notes document.I have an imported image in a richtext field in the document and am using the NotesDXLExporter to generate the file. The image has Alternate Text entered in the Alternate Text field of the picture properties box, but the picture tag in the DXL output does not contain the alttext attribute.

However when I add text in any of the fields in the HTML tab of the properties box the alttext attribute is available in the DXL output.

Is this a bug ? Has anybody encountered the same problem ?