This isn't a bug or an issue. When you place your finger on the left side of the screen it inputs that you are tapping the screen (which you are) and blocks. You then swipe to the left and move backwards. If contact is made before the swipe registers and the block is happening, you parry. The only way to fix this is if they added a slight delay in the listening to check if it's a swipe (like they do from the right side to determine if it's a light attack, a medium [swipe], or a heavy [hold]). If they did this, in the same scenario above, you would get punched or kicked in the face. We don't want that.
It's said that if you do the movement from the right side, this won't happen. But I understand it's hard to relearn how to play the game, so