Minecraft blend with shape key is not work with IK bones,but FK bones works well

I ran into a problem when trying to fix the bending of the limbs of Minecraft characters. I added shape keys to the limbs in FK mode and used the driver to switch between the shape keys and basic shapes. When I tried to bend the limbs in FK mode , Everything looks good, but when I switch to IK mode, the shape keys seem to have no effect at all. The surface of the model appears to overlap, as if it has not been dealt with. Does anyone know how to solve this problem?

Well,I have solved this problem,The method is to use the difference between the rotation of the upper bone and the rotation of the lower bone as the input of the driver.