is it normal?

sooo turret armor says ''immune to true dmg when no enemies minions nearby''

but i played yi and my e never did true dmg to turret i tried with minions around with no minions around my e active my e inactive in all cases it was the same dmg at same champ lvl and items

someone in game said it was normal and know for years... id like to know if it is normal or not before doing more tests(with other champs that can deal true dmg) and calling for bug fix