Steps to reproduce:
1.- Add a nested portlets portlet to a page (outer portlet).
2.- Add a Web Content Display portlet inside the nested portlets portlet (inner portlet).
3.- For the outer portlet, in Options --> Look and Feel --> Portlet Configuration --> set 'Application Decorator:Borderless'.
4.- For the outer portlet, in Options --> Look and Feel --> Border Styles --> Define a border style for the outer portlet.
5.- Refresh the page.
The inner portlet remains without any kind of border, obeying its own style definition.
The inner portlet inherits the border style from the outer portlet, even if no style was defined.
The issue is reproducible as well in 6.2.x, just changing step 3 by this:
3.- Set 'Show borders' to No for the outer portlet.