I Guess this is expected as you do not have a single instance where you can interact and set the values directly. You might have to the use the instances API in InstanceManager to set the field value of partular instance . The syntax to use the instances API in InstanceManager is:
<panelName>.instanceManager.instances[<instanceNumber>].<fieldname>
For example, you create an adaptive form with a repeatable panel having a text box. When you pre-fill the form with three repeatable text boxes, you need the xml below:
<panel1><textbox1>AA1</panel1></textbox1>
<panel1><textbox1>AA2</panel1></textbox1>
<panel1><textbox1>AA3</panel1></textbox1>
To read AA1 data, specify:
Panel1.instanceManager.instances[0].textbox.value
To read AA2 data, specify:
Panel1.instanceManager.instances[1].textbox.value