How do you determine the spot of a Gaussian beam defined by "ewfd - Gaussian field - Gaussian beam - plane wave expansion"? Or how do you keep it in a fixed position?

Hi, everybody, I want to simulate the motion of the microsphere in the optical tweezers system. I used "ewfd - scattering field - gaussian beam - plane wave expansion" to define the incident field, the spot of the Gaussian beam focus at the (0,0,0). But when I use "parametric sweeping" to simulate the position change of microspheres in the incident field, the spot of the Gaussian beam will also change. Why does this happen? It puzzled me. As shown in the multi-section diagram of the microsphere in the attachment, the spot position at (0,0,0), but when the microsphere moves to the position of (0,5,0), the spot focus is also around (0,5,0). Why does this happen? Sincerely, Miao Peng