Slabs disappering when tapped on

I was playing Minecraft on my iPad mini 4 signed into my Xbox live account and I was building a house with a slab roof. I accidentally tapped one of my slabs and it disappeared. I looked into it and I tried to do the glitch with other types of slabs.

Every type of slab has this glitch. I tapped the slab again with nothing in my main hand and it appeared again. I tried this on a device without an Xbox live account signed into and the glitch did not work.