Why is breakthrough voltage of a TVS-diode decreasing when in parallel with an inductance
TVS device clamping voltage is proportional to clamping current.

This excerpt from the datasheet shows this:



The highlight in red is the nominal break-over voltage at a specific test current (1mA in this case).

The orange highlight is the maximum peak current and the blue highlight is the voltage across the device at that maximum clamping current. Note that the clamping voltage can be as high as 70.1V for a nominal 51V device.

When you switch your circuit off, a large current flows which will decrease exponentially; with this variation of current, you will see a variation of clamping voltage.

Your plot is perfectly normal for this device.