This is an old bug and its been reported before, but its an especially egregious example of just how bad the quality control is at the Mcneel company.
Extending the edge on the enclosed model creates a Bad Object.
ExtendSrf_bug.3dm (49.3 KB)
But worse than that, Rhino fails to identify it as a Bad Object. After extending you can run DupBorder and then the Bad Object warning pops up. I’d like someone at McNeel explain how the borders of a polysurface can be a Bad Object but the Polysurface itself is deemed not to be bad?
This bug has been around ever since they made it possible to extend surfaces that are joined. That would be a nice feature if it could be trusted to not trash the users geometry.
So the prudent user has to continue to do it the old school way of extracting the surface then extending then joining it back to where it came from. If the user can do it correctly, why can’t McNeel?