not sure if this is a known bug or something. but lets say your a mage and you AA yasuo once to proc his shield and then say he dies to minions or tower or something neutral and not a champion. if he dies he will be executed. even though you hit his shield. a yasuo i played with said "because you didnt damage my health" which is fair enough. but then you can draw minion aggro and turret aggro from autoing and procing his shield passive.
This is normal behavior. If you proc his shield, which then wears off on its own without you breaking through it, and he goes and dies to minions/monsters or a tower, you did absolutely nothing to cause his death. So, you don't get credit for the kill.
There are some circumstances where it seems strange (you help to break someone's shield, but don't actually touch their health, then your ally finishes them off without no further help from you), but the behavior is at least consistent; if you didn't damage their health or apply any buff/debuff to the involved parties, you don't get an assist.
This is how all shields are: hitting the shield doesn't count as damaging the champion. This means you won't gain kill credit if all you damage is their shield and it also means you won't stop their back if you don't break the shield. Don't ask me why Riot made it that way.
yasuo is a broken champ
This applies to all other shields too.
