Instantiating prefab causes existing instances to change values

I want to instantiate obstacles so I have a controller to instantiate from a prefab over time, that is assigned through a variable. I instantiate the prefab like so (Instantiation over time works fine)

             Transform t = spawnX; // Transform to hold the x position of the spawn point
             Vector3 v3 = t.position;
             v3.y = Random.Range(minHeight, maxHeight); // Random y position
             t.position = v3; // Get the random spawn point

             // Instantiate the prefab
             Instantiate(obstaclePrefab, v3, Quaternion.identity, t);

Again everything is working fine, except that when one instance is created, all already existing instances update their y-Position which was randomly created to the one of the new instance. Why is that?