I have found the problem. I have got Rigidbody2D attached to spear. In this project I use my own physics, but I need Rigidbody2D for collision detection.
When object does not have any Rigidbody2D component attached, Bone Follower works as usual. However, when Rigidbody2D is attached strange stuff is happening.
After hit I simply destroy the component.
Destroy(GetComponent<Rigidbody2D>());
Thank you for your help Mitch.