This is the same on Live and Testlive, but the video is from Testlive so I thought it best to place this feedback here.
This is an issue with thralls being able to find the hit box for the Parasites. The hit box is very short and small and most of the thralls attacks miss the hit box entirely. I found the mace to be the best at finding them based on it’s attack pattern having multiple upward or downward strikes. I don’t think its as much a bug as just the way the 2 mechanics work or don’t work together for that matter. Seems none of the thralls are any good at baseball, watching them strike out over and over trying to hit the parasite is a bummer. Hopefully you can look into it and find a solution to the swing and a miss syndrome with thralls and parasites.