So I am wielding Phalar Aluve on Astarion in my current playthrough. He is currently spec'd into Assassin and gets to sneak attack anyone who's turn is after his at the start of a fight.
Everytime I go to sneak attack with the weapon no animation plays and the game just sits for a bit waiting for an animation that doesn't come.
The game eventually does continue but it takes longer than the animation would itself.
Not sure if this occurs with all Longswords or just this one.