Illaoi Gameplay. E sometimes breaks.

I just played an Illaoi game vs a singed top. I use e on him and attack the tentacle that is supposed to either last for 10 seconds while i beat on it or curse them when they leave its ranged. Instead what happened is after 3 seconds, the e spirit disappears and he is not cursed. This occurs when he does not run out of the ranged sometimes. no idea why it didnt last 10 seconds. this happened 3 times in a row. This is not the first time this has happened but with different champions.