A stone block occasionally harder to mine, then disappears

I will be mining and occasionally i’ll come across normal stone, and for some reason it’s harder to mine; as if it’s a bug and maybe should have been an ore. Once it’s mined, it just disappears. I can’t recall if it started at nether update or 1.14. Luckily it’s random and doesn’t happen too frequently. I’d say maybe 3 stones within an hr of mining (reg diamond pickaxe for example). 

I play bedrock edition (Mcpe). I’m hoping to record it some time as proof.