Elliptical flare is most commonly associated with anamorphic lenses. These cinema lenses squeeze the image horizontally so a wide scene can be recorded on a standard-size film frame and later unsqueezed in projection or post.

Because flare is caused by internal reflections, its shape often reflects the shape or orientation of optical elements in the lens. In most still-photo lenses, that means flares tend to look circular or sometimes take on the shape of the aperture blades. With anamorphic lenses, the anamorphic optics can stretch or bias those reflections, producing the characteristic oval/elliptical flare often seen in movies.

So the likely cause is not a special “flare part,” but the anamorphic optical design itself—especially front anamorphic elements and the way reflections pass through them.