I have no idea what happened, but in my creative superflat world in Java 1.16.5, I was using a normal oak slab, and it randomly changed into a 'Petrified Oak Slab'. It made stone noises.

I have never used any mods, data packs or shaders etc.

I did take out a normal oak slab in the first place, as there was never any petrified oak slabs available in the inventory.