Blocks disappearing when mined

When mining blocks, some of them disappear and are then not collectable by the player. My recorded video shows this:

https://www.youtube.com/watch?v=5RW9MUI-d4w

0:09 - Mined four Lapis Lazuli blocks with silk touch pickaxe - 1 disappears.
1:10 - Two gold blocks mined with efficiency IV, unbreaking III pickaxe - 1 disappears.