Slabs and glass no longer prevent daylight damage to hostile mobs

I'm not sure if this is an intended change, but slabs and glass no longer prevent hostile mobs from burning up in daylight. I'm just doing the zombie cure, on one of my villagers, and he went up in flames as his roof is just a bottom slab. A named zombie was under a glass walkway and he too died of sunburn.

Now I'm wondering if it's always been like this, but I remember mobs hiding under a slab and stair porch and not burning. Now they are burning, even when the stair is on top of a glass block.