Turret glitch

So is following me under my turret with his E active (im just out of its range). I'm playing as and I turn around to use my ultimate on him. He teleports to me and does his E dmg. Immidiatly after that I start channeling my ultimate on him. He is directly under my turret, so he is most definitly in turret attack range - but he still doesn't get targeted by the turret. Is there any way you coluld explain this incident? Or was it a glitch that needs to be fixed? Looking forward to your answers.