From user Forum [https://forum.unity.com/threads/2-6-0-preview2-2d-confiner-jerking-on-bounded-edges.857368/] :

There is a regression that if the VCAM is following a target character and it reaches the edge of the bounding box it will no longer just stop moving (as expected with a damping of 0 on X,Y,Z), but instead start jerking.

You can repro the problem by importing the Cinemachine Example Scenes and loading the 2DConfiner scene. In the CM vcam2D menu, change  X, Y, Z Damping to 0 in the Body - Framing Transposer. Hit play, move the character away and come back near the wall ==> Notice the camera is jerking a bit when away and back to the wall.

This bug is also in FogBugz with ID 1232146