I get three shots, i dont think its a bug because imagine if you shot accross the map and hit 12 shots into an enemy... unless the spread is high enough to scatter all 12 shots the enemy would be dead in 1 hit with a shotgun...
like i said, i get 3 shots per trigger squeeze... not sure if its a bug, i think its an implementation to ensure smooth gameplay (it isnt halo standalone - its halo minecraft.)