Not sure if you got this working or not, but here is a link to another post that shows how this works.

https://community.nintex.com/t5/Community-Blogs/Using-Javascript-to-update-a-repeating-section/ba-p/82584

Go to form settings and under Custom JavaScript add the following code:

NWF.FormFiller.Events.RegisterAfterReady(function () {
 NWF$(document).ready(function () {
 NWF$("#" + varDropdown).change(function () {
 //clear repeating section
 ClearRSData();
 //get repeating section data
 GetRSDataFromLookup();
 });
 });
});

function ClearRSData() {
 //get the repeating section
 var rsdata = NWF$('.rsdata');
 //count number of fields to give number of rows
 var labels = NWF$(rsdata).find('.dwgDimension');
 //iterate through rows
 NWF$.each(labels, function (index) {
 //delete the last row
 NWF$('.rsdata .nf-repeater-row:last').find('.nf-repeater-deleterow-image').click();
 });
 //for last row as index 0 row cannot be deleted
 NWF$('.rsdata .nf-repeater-row:last').each(function () {
 //get the row
 var $row = NWF$(this);
 //clear the field value
 $row.find('.dwgDimension input').val('');
 //clear the field value
 $row.find('.ddOperation input').val('');
 });
}

function GetRSDataFromLookup() {
 //get the lookup value
 var lookupId = NWF.RuntimeFunctions.parseLookup(NWF$('#' + varDropdown).val(), true);
 //if lookup value is not null
 if (lookupId != '') {
 //clear jQuery error messages
 NWF$('#errorMsg').empty();
 //set the lookup list url to get repeating section xml data
 var requestUri = _spPageContextInfo.webAbsoluteUrl + '/_api/Web/Lists/getByTitle('ListA')/items?$filter=Title eq '' + lookupId + ''';
 try {
 NWF$.ajax({
 url: requestUri,
 type: 'GET',
 headers: { 'ACCEPT': 'application/json;odata=verbose' },
 success: GetRSDataFromLookupSuccess,
 error: GetRSDataFromLookupError
 });
 }
 catch (err) {
//set error message
 NWF$('#errorMsg').html('getListData Error: ' + err);
 }
 }

}

function GetRSDataFromLookupSuccess(data) {
 //check the result set is not empty
 if (data.d.results.length > 0) {
 //get the xml data from the result 
 var rsdata = data.d.results[0].rsdata;
 //intiate xml parser
 parser = new DOMParser();
 // populate the parser with the xml data
 xmlDoc = parser.parseFromString(rsdata, 'text/xml');
 //get the item tag from the xml
 var items = xmlDoc.getElementsByTagName('Item');
 //foreach item in the xml
 for (var i = 0; i < items.length; i++) {
 //get the repeating section last row
 NWF$('.rsdata .nf-repeater-row:last').each(function () {
 var $row = NWF$(this);
 //set the field value
 $row.find('.dwgDimension input').val(xmlDoc.getElementsByTagName('dwgDimension')[i].childNodes[0].nodeValue);
 //set the field value
 $row.find('.ddOperation input').val(xmlDoc.getElementsByTagName('ddOperation')[i].childNodes[0].nodeValue);
 });
 //add new row
 NWF$('.rsdata').find('a').click();
 }
 //delete extra unnecessary row
 NWF$('.rsdata .nf-repeater-row:last').find('.nf-repeater-deleterow-image').click();
 }
} function GetRSDataFromLookupError() {
 alert('Failed to load Lookup Items');
}‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍