Relic Shield, with charges ready, not executing minions under 200 health on basic attack.

I joined a game today as support Thresh and I started with the first stage Relic shield and it was not executing the minions I hit with a basic attack. I clicked on the minions to check there health and they were well below 200 health. Also I checked many times to make sure there were charges ready to be spent and no matter the situation, the minions were not executed unless my regular basic attack damage killed them. Then and only then would it spend a charge and give me and my ally the health and the coins from the minion.