Burning Kelpblock-roof in spite of using a lightning rod

I was coming back from a greater distance, while a thunderstorm begun. As I reached my village near the spawn, one roof of my fishing huts whas destroyed. 

The good news of this - the lightning rod on this shack was still alive! Only the kelproof was destroyed.

I am not sure, this happend in 20w48a or 20w46a. I don't see the fire, only the destoyed roof.