The purpose of Dense infill is to enable regular infill to be as light and airy (fast to print, consume little material & easy to break away) as possible. To effectively span and fill the gaps at the top of such a regular infill may require many layers of dense infill (eg 5 layers). Because at first, due to gravity, it will tend to sag through the gaps.
When dense infill is applied UNDERNEATH regular infill, however, gravity no longer pertains and it is necessary to use far fewer layers of dense infill to achieve the same effect. And indeed if not using a soluble support material for the dense infill, perhaps wouldn't want any at all undernath the regular if it's set on a nice flat top layer.
SUGGESTION: enable the setting of dense infill layers separately for ABOVE and BELOW the regular support.