It seems that stairs are the same as glass that cannot shade zombies against the sunlight to prevent zombies from burning. Why I regard it as a bug is that zombies will burn under a stair, which amounts to three quarters of a block, but not burn under a slab, which amounts to a half of block. It is unreasonable that stairs contain more substances but have less ability to stop light. I hope this bug can be fixed as soon as posible.