Not an expert here,
But I suspect that this shading change result from a lost of continuity in the wing, and that give some trouble to build the fillet.


Looking at the profile curve of the wing, there is indeed probably a discontinuity in this area, the 3 points (red lines) should fit on one line.


So with the help guide, I draw a line going throuth the 2 first curve control points


and move the third one on the same line


I don't really get how you build the wings, so I reuse the 3 curves in the file, using the network ... seems to give something pretty close from
the original one


Now it's time to clean this part, separate everyting and delete the wing


Select the body and click on the show points command


Select the hole edge curve


and hit delete.


rejoin everything.


Don't forget to cap the wing with the make planar command


Booelan union


Fillet should work !


Sorry for all the spelling mistakes, did it really fast ;)