Hi,
there isn't any automatic binding between a playerpref key and what controls it ( your prefab), so you need to create a key yourself.
typically, you will need to do the following:
1: when you instanciate a prefab, you need to give it a unique name or id,
2: when you want to store the player position, you create a key that starts with the prefab name or id + "-position" so you would have "Prefab1-position"
3: then when set get this key from the playerprefs, you can't get mixed up between prefabs.
does that make sense? the playerPrefs is a very raw system, a list of key/value pairs, so you need create your keys very carefully so that they are unique and don't overwrite one another.
bye,
Jean