The rectangle needs to be there - this is a fiberglass molding plug. The goal is to make an indentation the shape of the wing in a flat body surface. You cut a hole in the existing surface, attach this to the outside, and lay new fiberglass into the hole, resulting in a flat surface with a recess in the shape of the wing. So the rectangle has to be there to provide the surface.

I tried uninstalling Cura, re-downloading and reinstalling, and checked the printer specs (height = 400mm) and got the same result.

So I think this is a problem with the .stl file.
I can model the shape in Solidworks to make the wing part integral with the rectangle instead of being an add-on and see if that helps.

I'll try printing a 25mm cube first as a test.