The spline for the second primary curve is not good.
If you create a section curve on the extrude at the start of the spline and measure the angle it is not 180 degrees.
Also if you look at the poles of the spline the second pole is very close to the start of the curve and the first pole is not exactly on the start of the extrude edge.
Hi @StevenVickers Thanks!
Yes. Those imported objects have some problems. However, what confused me is that to simply exchange the primary curves and the cross curves would make the surface able to be created successfully.