Please only use one override region for the coupling. The waveguide will use the mesh accuracy to be well resolved. When you used finer mesh, it may affact PML thickness.
From the image, it seems no strong fields to the bottom-left port, which is a good sign.
As you can see, there are some scattering fields after the coupling region, and they may have larger angle to reach PML. This is why I suggest to check the mode profiles. You may need to use 64 layers for x-max PML or even use Steep Angle PML with more layers.
You can try to simulate half the device, eg, move the right-hand-side PML to the half way of the coupling region and check if it diverges.
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