In the the 1.64 was added bug: If the stone is in front of the player and this player want to lie on the ground, then, he can not do it normal. In this case, all you will get - is a look into heaven in a not natural body position.
Probably was the idea, to create the effect of obstacles for weapons, but the result, with stones is turned out, not good.
The problem looks like the collision is considerably larger than the object to which this model belongs.
At a distance of 1 meter weapon collides with an invisible barrier. In addition to this human body takes the position of the snake: https://www.youtube.com/watch?v=Sgt8IYEfeVM&feature=youtu.be
As it turned out, the problem appears not only near the stones:
Thanks Lex for additional example: https://www.youtube.com/watch?v=1JWXegeMKPU&feature=youtu.be
It also appears to have low fences or low borders. Please correct it, because it irritates the player, especially in moments of battle.