1. I was able to create the extapolated surface by choosing the displayed boundary and tangency continuity of the propagation mode of the extrapolate command as it was expected.
2. With the same parameters except the prompagation mode (point continuity now), got a twisted surface!
3. With the same parameters as No.1 except the boundary, got an extrapolation suraface as it was expected.
It's strange for me. What abou you?