The underdress only theory is that between the smock/chemise and the Burgundian gown is an underdress. I’ll call it a Kirtle, some people call it a gothic fitted dress. Back in the day this was known as an underdress theory as there was debate on if there was a dress worn under the Burgundian gown or if it was just a placket directly over the chemise. These days it is generally agreed that a lady would not wear the gown directly over her chemise so the theory focuses more on what ELSE was worn under the Burgundian gown.
Support for the Chemiseà Kirtle à Gown theory can be seen in any painting that shows the undergown’s hem being the same colour as the v of fabric showing under the neckline of the gown.