This is because 2D is mostly unphysical.
The example I provided has the comparison. Please refer to that example and see if you can improve some settings. Please note that STACK is analytical solution and FDTD is discrete solution. the latter needs careful settings such as mesh accuracy and simulation region, as well as PML.
Please make sure the dipole orientations are the same for STACK and FDTD. 2D simulation will still need 3 dipoles, and different locations as the dipoles must be in the active layer.
Dipole radiation power is very sensitive to locations and relative to the mesh.
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