Q277 roguish ASSASSINATE says
Starting at 3rd levei, you are at your deadliest when you
get the drop on your enemies. Vou have advantage on
attack rolls against any creature that hasn't taken a turn
in the combat yet. In addition, any hit you score against
a creature that is surprised is a criticai hit.
Could creature be surprised more than once per battle? If rogue sneak, running to different direction and hit from new position, for example.