Why can't the weight of IK constraint be keyframed with Animation Rigging package?

I have a rigged character (biped structure from mixamo) with a generic rig. I have nested it (mesh and bones, basically the .fbx prefab) in "Character" game object so that the Character holds all the components such as the Animator and other scripts.
The Animation controller has an idle animation that is working fine when the game is run. I have set up the Animation Rigging package so that I have added 2-Bone-IK constraints to both hands. Constraints work fine in Play mode. I am trying to create an animation clip keyframing the positions and the weights of the IK constraints so that it'll add up to a weapon holding pose on top of the idle animation.

In the keyframe recording mode, I set the 0th to have the IK Weight=0. Then dragged to the next frame and set the weight for the same IK in the same target to 1. At this point, even the 0th frame resets to Weight=1 so that both keyframes have Weight=1. Besides, the position changes of the targets are saved in keyframes as intended.

But to me, it's crucial to bring the Weight value to the clip.

Please help me figure out whether I'm doing anything wrong or what things that I need to look at.