It's probably not a bug, just a questionable design decision. If this is a bug, its been in the game for YEARS. The assist policy on shields is simple: if you damage a shield, and that shield is then BROKEN (by you or someone or something else) then you get assist/kill credit. If you damage a shield, and that shield EXPIRES by timing out, then you do not get assist credit; doing as much damage as you did when you did made no difference. In this case, I suspect the slow-mo would show the shield expiring immediately before the tower shot hit Yasuo.
I would argue that you SHOULD get assist credit in this specific situation because you did CC him by knocking him farther under the tower, because its also the policy that applying non-damaging CC to someone will give you assist/kill credit. But idk.
Actually now that I think about it, this totally IS a valid bug and should be explored further. Upvote from me.