How deathfire touch applies damage seems weird, is this how it's supposed to work?

I'm using rumble in the practice tool and noticed that deathfire touch's damage resets if a different ability is used right after. If I use E then Q right away the E 4 seconds will immediately cancel. However, if I wait the four seconds, then use Q, the full damage is applied. Without any items the difference is like 20 damage, which is negligible. However, at level 10, with 4 items picked the difference in damage is well over 500.

Is that how it's supposed to happen? I suppose one can time the abilities properly but, in Rumble's case, the whole point of the harpoon is to apply the slow, not to wait 4 seconds and then use Q.