The waveguide is ridge-type, so it is very dispersive between TM and TE. The example is designed to work for one polarizatio, as it used "anti-symmetric" BC on ymin. Please check the mode profile of the injection port before the simulation, and make sure it is the mode you want.
Once you change the polarization, you have to modify the BC. please refer to this article: https://optics.ansys.com/hc/en-us/articles/360034382694-Symmetric-and-anti-symmetric-BCs-in-FDTD-and-MODE
If you have questions for the symmetry BCs, please write a new post.