My bones with the inverse kinematics applied just don't rotate/translate properly:
I would like them to stick together at the knee with the knee bending accordingly. Also if I set the higher leg as a parent I have this result:
I also tried to connect them with ctrl+p in edit mode but nothing changed. I know that if I make the bones from scratch extruding from the tip of the bones I would probably solve the problem but I want to understand this behaviour.