Transient voltage suppression diode not doing its job
Very often the parasitic impedance between the lines and diode is some nanohenries. This caused by the length of diode leads.

This impedance slowing the response of TVS diode by nanoseconds, thus the transient pulse can pass.
You can use a TVS diode with so called “flow through design” in which you have to pass rail and ground through the same package.

Of course this needs a proper design of  PCB.