[GAMEPLAY] Minion sudden death bug

While trying to farm, for the sake of description I have been in top lane, suddenly a minion with around half health will drop to zero at the exact moment I target it, but before the full attack animation goes off resulting in me not receiving the farm gold. This usually occurs during lane phase when both enemy and friendly minions are in the lane fighting. I will go to target a minion with a decent amount of health and once I click on it, it will suddenly die before my full attack animation finishes. There is nothing around the minion that would cause that amount of damage and it seems to me that there is a slight bug when a champion targets a minion at the same time a friendly minion's attack hits the targeted minion it will lose all of its health. I'll try to make it clearer. An enemy minion that is being attacked by a champion at the same time as that champion's friendly minions will suddenly lose it's entire health bar and does not result in gold given to the champion. When this has happened to me I have been in the top lane with both minion waves attacking each other during early game with a max of 2 champions(one from each team) in the lane. I have targeted a minion that is seemingly low enough to kill for gold, but as i click on it, it suddenly loses a large amount of health(too much for a opposing minion to deal) causing me to not receive the gold for the minion kill. Please let me know if there is something that you do not understand.