As the title says the Coins and other things in coin river can tend to despawn.
To repeat this bug you play the arcade game (Coin River), then leave the arcade and then come back to the same machine. Then everything that was previously on both rows will have despawned.
Before starting the game when you have done as said above, no coins/bars/barrels will have spawned. When you start the game it will reset and spawn coins/bars/barrels but only on the top row.