Slabs seem to disappear when restarting the game and then entering the same world, but the bug doesn't seem to occur if you don't restart the game before re-entering. Updating the block where the slab was will make all the slabs reappear which means they didn't fully disappear, they just didn't load. Sometimes they even become Stone Pressure Plates for some reason, but the hitbox of the slab is still intact

Only tested with Quartz Slab, please confirm if the bug also applies to other slabs.