[GAMEPLAY] Sivir with spellshield still stunned by Veigar E

If this is the case then it is still inconsistent. In that same game, the Sivir later manages to walk through the wall with spell shield.

No, this is incorrect. You're calling on-hit effects as "passive" I'm assuming (i.e. Wit's End, Guinsoo's, Liandry's, red buff, etc.).

However, the event horizon is definitely something that should be blocked by the spell shield.