oh that's normal. It's a counter attack. If the enemy has 0 defend points and you parry their attack it'll trigger a counter parry which you can then follow up with a downwards slash.
333 days ago(+1)
After further experimentation on this... it's only possible on the final hit of the 3-hit combo, that thrust is the only one you can counter.
331 days ago(+1)
It's currently supposed to be like that so you don't just interrupt the enemy's combo on the first hit immediately. Might change it later on to be more consistent though.
That makes sense. If you can only counterattack off the final hit in an enemy's combo, that's already consistent. It's just consistent in a way players will learn over time. A lot of games have mechanics like this, or ones where you can counter some enemies anytime, but others only off certain attacks.