Instances are duplicates of same mesh. If you displace this mesh, all instances will continue to have the same shape. You can randomize scale, translation and rotation of each instance but you can not randomize data that is instanced.
You have two solutions.
You can use Realize Instances node to create a global mesh and randomize its vertices.
Or, you can use several displaced meshes, create one instance per mesh and join those instances as one geometry (or use different objects placed in one collection) and, then, instance the geometry group or collection with an Instance on Points node using Pick Instance option.
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