It's intended behavior known as "follow-thru". Sedna has some of the worst follow-throughs in the game, which contributes to her already absymal dps.
Other characters don't feel it as much because their alpha strikes are tied to a snare (Bite, Pentitence) or they are just naturally fast.
The reason it exists is purely graphical and it avoids a few race conditions. I proposed an overall of the follow-thru system and reducing follow-thru for all abilities to something small like .25 but it was cut down as a "balance change".