Just tested this in the Practice Tool but don't feel like making a video cuz Riot aren't gonna actually read this and fix it so w/e
Bug: Illaoi's passive is unable to spawn tentacles inside of friendly T2 towers while the T1 tower in that lane is still alive, and therefore the T2 is invincible and "protected". Once the T1 tower dies, you are again able to spawn tentacles in the T2 tower. The same is true for inhib turrets and inhibs: can't spawn tentacles in them until they are "vulnerable".
Strangely, you ARE able to spawn tentacles in protected ENEMY structures. Just not friendly ones.