same thing has happened to me with different champions , once they enter the brush ( summoners and ARAM ) they can become untargetable even though you can still see them , then they disappear from your sight even though you followed them in to the brush and then they kill you while invisible only becoming visable just before you die.
They should make it if you move while in the brush enemy can see you. if your stood at the edge of the brush you can still be seen you only become stealth if your in the centre or to the back of the brush.
I have been killed many times by players who only need 1 more shot to be killed and just because they got to the edge of the brush and I could not target them even though I could still see them.
But because I was not in the brush they could target me and hit me with everything they had I could not do a thing about it even though I could plainly see them and they was well with in my attack range but I could not attack them because according to the game they was invisible making them untargetable.