Why is this happening? Bad brewing technique or bad coffee?

I am making a v60 with 20grs of coffee and 300ml of water. I am doing the 4:6 technique and usually get good results, but sometimes this is the result and it comes with a bitter taste.

I am making a v60 with 20grs of coffee and 300ml of water. I am doing the 4:6 technique and usually get good results, but sometimes this is the result and it comes with a bitter taste.