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Not entirely certain if this is intentional or not... I would hope not.
Anyway, if Fiora attacks an enemy champion under the turret with minions nearby, then ults, the turret will stop attacking her, even once her ult ends (obviously it can't hit her WHILE the ult is going). I witnessed it myself when a Fiora dove our solo top. Turret shot her once when she chased our guy under his turret, then she ulted, then she walked past the turret (in range all the while) as it shot at minions.