[Gameplay]Thresh AA not triggering Relic Shield execute passive

When i use thresh, and i buy Relic shield as my starting item, i go to hit something below 200 hp and the passive dosen't trigger. If i kill the minion with regular damage i still share the gold, but the <200hp execute never kicks in.

1. buy relic shield
2. AA minion <200 hp but >standard AD
3. no execute

its occured 3 times, for the entire game.