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.
You grabbed the petrified oak slab instead of the oak slab in the creative inventory