while shrinktrimmedsrftoedge command will work as expected.
Try on the attached:
import rhinoscriptsyntax as rs
filter = rs.filter.surface | rs.filter.polysurface
surface = rs.GetObject("Select surface or polysurface to shrink", filter )
if surface: rs.ShrinkTrimmedSurface( surface )
shrinkfacesfail.3dm (38.3 KB)