When blocks are broken near the player, they sometimes (near 20% chance) appear into the player's inventory without a pickup noise (The 'pop' sound) and without actually dropping as a visible item (an item entity).

Usually, the blocks break and you see an item entity and then when walking near it, you pick it up and hear the pickup noise.

What actually happens is the block is broken and occasionally (about 20% of the time) the item appears in the player's inventory without a pickup noise and without an item entity appearing.