It is good to only override mesh on the thickness, one dimension. If such thin layer does not primarily determine the mode profile, you can try to disable it first, in order to find the diverging cause.
You need to check where the scattered fields go. If the scattered fields also go to ya dn z, you should not use metal BCs.
You can use 3 rofile monitors in xy,xz and yz , and use log scale to check the scattered field intensity. If they are large, say 1e-4, you will need to use thick PML.
"Is it just for checking the divergence to move the PML right end to the center of the couple region? "
yes, to find the cause of diverging.
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