For some reason I can't add an image, sorry.

I drew a rectangle, added an arc to one side. Trimmed the rectangle so three sides are straight and at right angles to each other, deleting the 4th side (where the arc is). Joined the three parts of the rectangle to the arc so I have a closed curve (verified by Moi info in upper right corner).

When I select the closed curve and extrude it, it extrudes as a rectangular solid, it ignores the arc. What's going on?