Limiting voltage transients with a TVS diode
The datasheet specifies 58.9V at 1mA, the very beginnng of conduction, but voltage can be as high as 77.4V (maximum) at 2.6A. Maybe the energy of the spikes generates more than 1A transient current. 

What other loads are connected to the 48V line?

A possible solution would be using a dedicated cabling from the power supply to the regulator, possibly including ground, so no heavy current changes circulate thru the cables.

Another one: add more external C and if possible some series inductance in the cable from the supply. The datasheet mentions "If you are connecting more than 28 V or your power leads or supply has high inductance, we recommend soldering a suitably rated 33 μF or larger electrolytic capacitor close to the regulator between VIN and GND". Do this little by little, sometimes adding an integrator like that will slow down the peak voltage but may increase the peak due to sub dampened "resonance".

Yet another hint: place the TVS from collector to base of a NPN high current transistor, 100 ohm from B to E, and connect its C end E in parallel with the power lines. Effective clamping voltage will be reduced because hfe times less current will pass thru the diode.