I’m pretty sure, it’s because he got attacked from other players at the same time.
You can interrupt his attacks in the assassin-form with continuesly attacks of all party members since the dlc-patch. As you can see, your slayer attacked him so fast, that the attack-interrupt-animation interrupted the downed-animation.
A console.log could be useful for FS, but i’m pretty sure , that my supposition is correct.