Extending a natural surface with ExtendSrf, Merge off, and the Smooth option results in a trimmed surface. See the CP layout of the extended surface. In my opinion, this is a bug.
Every now and then, it makes the bottom edge a trimmed edge, so the bug isn’t even consistent; the right edge is always a trimmed edge. With this bug, resulting in two trimmed edges, I can’t even use the also defective RefitTrim tool to obtain a natural surface.
@Lagom
I don’t know. I am not aware of EdgeSrf creating trimmed surfaces. If you can reproduce the creation of this with EdgeSrf, pls send a file.
Edit: I did a quick code investigation, and it doesn’t seem possible from the code, that the command produces trimmed surfaces. Were other commands involved before you noticed the issue with ExtendSrf?