BEAM188 is a 2-node beam, so is a linear element.
BEAM189 is a 3-node beam, so is a quadratic element.
When you keep the element order Program Controlled, that generally creates quadratic elements. Look in the Solution Output and you may find that BEAM189 elements are used. What evidence do you have that BEAM188 elements are used?
For a single element, the quadratic element is going to match the analytical calculation of a cantilever beam. When you have many elements, the linear elements will also match the analytical calculation.