Yes, on an element-by-element basis, the material response is linear. The stiffness of each element is linear.
But on the global scale of the entire structure, the force is not linear with tip displacement because the stiffness of the structure is a function of tip displacement caused by large rotation of the elements.
The reason is that the elements experience a large rigid body deflection (and large rotation) from their initial coordinates. This is large deflection nonlinearity. But the local deformation of each element remains linear.