This happens to me after 1.12. upgrade... when harpy is attacking at Geralt, it always falls down on the ground, Geralt pierce it with sword, and harpy dies immediately. Every one corpse then after death jumps up to the air and moves to side a little bit. Every one harpy has the same animation.

I'm not sure I'm understanding but,

after enemy death even with drowners and neckers and such, the physics after the death animation kind of pop in causing the bodies to move like that. I actually like it because I'm a fan of physics/ragdolls and though its more minimal in this game, it shows me that the physics are still there.

Now the harpies, if you are blocking, sometimes the harpies will be knocked to the ground after a good block or parry where upon which you can initiate that instant kill animation where Geralt stabs them when they are on the ground.

I'm not sure I answered the question but these things have always been in the game as far as I know and that I've witnessed.