230VAC dimmer MOSFET failure at surge test
The main issue here is ( no pun intended) the lower main line injection of 0.5kV with a 1us rise time and Miller capacitance coupling of impulse voltage back to the gate drive is probably exceeding the Vgs levels. This rise time equates to a the driving spectral energy band of \$f_{-3dB}=0.35/t_R= 350kHz\$

You have a CM choke on 1 side but not the other. It needs a CM choke between both lines to raise the CM impedance and a differential choke to raise each line impedance due to lightning rise times.


These can be SMD CM chokes types with sufficient voltage rating. 
You can also add 500 Ohm ferrite beads at 350KHz.
e.g. https://www.digikey.com/product-detail/en/tdk-corporation/VFS5045VA111/445-181417-2-ND/9817660