Question Constraints broken after changing Unity version

My problem is that I am changing weights of rigs (I have 2 rigs for my character) using script. When I activate a rig by increasing its weight, it gets activated but the bones are all messed up at wrong position, but after a few seconds, they SUDDENLY come to where they should be.

I had set up a third person shooter controller in Unity 2021.2.1f1, and everything was working fine. I wanted to go to 2020 LTS since I realized my mistake of not having the stable release.

I thought it was my character's issue. So I reimported my base model into the scene, set up the entire thing all over again, with fresh new constraints and targets and everything. But the problem persists.
What's strange is that the bones ULTIMATELY DO COME to where they should be, but for many seconds they stay messed up. It's very annoying.

(Note: I am only using Multi-Aim-Constraint and Two-Bone-IK-Constraint)

Any help is appreciated. Thanks