I know I've mentioned this, many months ago.
Some enemies throw rocks that do not leave a trail in the air. Okay, that mechanism is hardly realistic but it's inconsistent.
In the video (yes, me posting a second video) it's a bloke with a flail. Three times in quick succession. Also shows a trail from a Standard Bearer's spear and the trail from the rock when Eivor throws it back at the bloke-with-flail and kills him.
https://gamerdvr.com/gamer/spirantcrayon22/video/151062676