I was playing around with some Petrified Oak Slabs when I found that I couldn't burn it with a flint and steel. I then stacked the slabs and found that I could burn it. I thought it was weird so I got a normal Oak Slab and tried to burn it and it was successfully put on fire. While the Petrified Oak Slab was unable to be set on fire with Flint and Steel when it is not stacked. I provided an image and a way to replicate it down below.

Replicate

• Get a Petrified Oak Slab
• Place Petrified Oak Slab
• Get Flint and Steel
• Right Click with Flint and Steel on Petrified Oak Slab