Zombies under a stair will still burn in daytime as if they were directly exposed to the sun

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.