[GAMEPLAY] Execution after damaging an enemy's shield

In my most recent game, I was turret dived by a thresh/tristana in bot lane. During the dive, the Thresh used his lantern to shield himself, then flashed out - but not before I hit him with an auto attack. The auto attack didn't eliminate his shield, but it did do enough damage for the final turret shot to kill him through the rest of the shield. Despite that however, it was considered an execution. I can't seem to find any information asserting that executions when damage is only applied to a shield is intentional, nor does that seem reasonable, as it can make the difference of an enemy surviving a dive such as this.

Attached is a replay - hoping for an explanation or fix :)

On an unrelated note, I also had an issue at the end of this game where none of my keyboard keys were responding, whether for typing, summoner spells, or abilities, even after alt-tabbing. Wasn't able to determine the cause, but if someone else has something similar, might be something to look out for.