Steps to recreate issue:

1. create a world on bedrock edition
2. break one block
3. place a two block pillar adjacent to the broken block
4. fill the hole with water
5. place a slab in the hole i.e water lock it
6. drop sand on it with help of pillar adjacent to it 

You will notice that it has disappeared not even dropping the item.

I don't know if this is happening to all bedrock platforms but I found it on my mobile i.e pocket edition