Ok, I don't know if its a bug or not, or it is supposed to be this way.
It was not like that before, I play for like couple of months. When I am blocking with a shield and holding the right mouse button constantly , the shield will drop down (go out of blocking position) for a second. That can happen few times in 10 seconds. It is very annoying... and I tried different shields. My player is lvl 30, Shield 6.
thanks