This is one of the most recurrent question regarding subdivision with trimmed or untrimmed surface.
See the link bellow to understand the underlying theory.
And I can show you one workaround way on this using Copy Trim and Surface Morph.
http://docs.mcneel.com/rhino/5/help/en-us/seealso/sak_splittrim.htm
http://docs.mcneel.com/rhino/5/help/en-us/index.htm#commands/untrim.htm
SubdivisionTrimmedSurface_re.gh (33.0 KB)