This jump back happens because the collision detection is now limited to just around the camera. So, if the camera is above the staircase rail, the collision with the rail will not be detected, and the movement through the rail will be allowed.
We are aware the collider can be improved, adding a full-body collider is mentioned in this Wishlist post.
For now, in places like the staircase the collision behavior can be improved by introducing fully-transparent vertical panes that block the movement. We will shortly write a short post about it.