Curves cannot form a surface using Loft

Hello. I want to achieve something like the first image, but after I used graph mapper to alter the curves’ points, they cannot form a surface any more. To be specific, it is the bottom curve which is not connected. I would like to know the reason and your help is very much appreciated! Thank you!

Flatten the input as you have two branches.


Thank you very much! It can now form a surface.