If you turn on Zebra stripe you can see that the surface does not follow the curvature the whole way across the seam.
If you thinken before performing the loft, click on the 3D sketch, the edge of the curve and then the face, it should open a mate allowing you to match the curvature. I am away from my work computer today, otherwise I would upload the result.
what was the reason that you did individual thickens near the end of your part. I rolled back to the tab trim feature and knitted all four of your surfaces at once (and checkmarked the 3 gaps) and then did a thicken. This eliminates the gap you are seeing.
I had originally knitted them and tried to thicken them all at one time but I got an error message. That's the reason for the seperate thicken operations. I went back after reading your post and tried it again with checking the 3 gaps and it worked fine. As I said, I'm just starting with surfacing and I have a lot to learn. Thanks for the help.
The 'flat' surface in your loft is distorted. This can be cured by replacing the central guide curve by two at the outer edges of the flat surface.
Zebra 1 is your original design, Zebra two with the added guide curves.
See also a simplified construction with surfaces knitted together and then thickened (pactice loft v2).
Hope this helps