What "time scale" do you mean?
AnimationState timeScale and TrackEntry timeScale do not affect physics speed.
What are you passing for delta to skeleton.update(delta)? If you scale the delta to slow down physics, then they will slow down all the way to not moving when delta is zero. Otherwise they will spring as normal. You may want to limit scaling this delta to 0.5 or 0.25, this way the scaling has an effect but never reaches zero and never gets low enough to see jitter (assuming you started with the default 60Hz update rate, see jitter below).
If you don't scale the skeleton.update(delta) delta then your physics will spring back at full speed, regardless of anything else. If you also slow down your animations (with either timeScale mentioned above) then the physics spring forces will appear stronger because they are returning to the unconstrained pose at normal speed, but over time the pose is only changing a fraction of how much it would change were the animations at full speed. The result is that it will appear that the physics forces are stronger (ie they return the the unconstrained pose more strongly, so you get less of an effect from physics).
If you do scale your physics delta, then the physics ticks will happen less often. At slow speeds this starts to become noticeable: the skeleton moves for some time, then the physics adjusts it, causing jitter. This is the same effect you would see if you reduced the physics constraint update rate (FPS).
If you want to scale your AnimationState or TrackEntry timeScale to play slower/faster animations and also scale your physics delta to apply less forces to match your slower animations, then to counteract jitter from a low update rate, you can increase the update rate. For example, if your timeScale is 0.1 and your delta is multiplied by 0.1, then also multiply PhysicsConstraintData step by 0.1. This way you get physics forces appropriate to your 0.1 time scale and a higher update rate to keep things smooth.
