This is a common question regarding to "sinusodal flutuation" in transmission/reflection. The common cause is, the simulation time is not long enough, or the autoshutoff min is not small enough. Please significantly increase the simulation time, say 100 times more, and let the autoshutoff min to terminate the simulation. You can use the "check point"  https://optics.ansys.com/hc/en-us/articles/360034382534-FDTD-solver-Simulation-Object to reduce the autoshutoff min and resume the simulation to improve the result, which is much more efficient.
You have chosen the steep angle PML with 36 layers, which is good. However, such device may have strong scattering at larger angle, so please use more PML layers. It is important when you used finer mesh. Make sure the PML is thick enough, say half wavelength.
Finally, maybe you have already did so, but I still suggest to check if the SiO2 substrate has been extended to outside of PML. Otherwise it will create oscillation due to cavity effect.
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