I had similar issue before, but that was when using loft from two closed curves.
The surface was a bit twisted. Used simplycrv to make the close curves, but still same.
This time, the loft makes a good surface, but when I try to join two surface created by loft,
the output surface becomes a bit twisted.
So making a closed curve -> loft -> then join. or boolean.
attached is rhino file, and some screen shots of analysis tool.
using WIP build 5B148w
question_loftandjoin.3dm (135.6 KB)