Second <aura:iteration> not working with object/array datasource

I have a Lightning component with two <aura:iteration>

<aura:handler name="init" value="{!this}" action="{!c.doInit}" />
<aura:attribute name="invoices" type="Object" />
<aura:attribute name="invoiceLines" type="Object" />

<aura:iteration aura:id="invoiceList" items="{!v.invoices}" var="invoice">

  <c:lightningFormElementOutput label="Invoice ID" value="{!invoice.Name}" />
  <c:lightningFormElementOutput label="Person" value="{!invoice.Person__r.Name}" />
  <c:lightningFormElementOutput label="Status" value="{!invoice.Status__c}" />
  <c:lightningFormElementOutput label="Total" value="{!'£' + invoice.Total_Amount__c}" />

</aura:iteration>

<h3 class="slds-section__title">
    Invoice Lines ({!v.invoiceLines.length})
</h3>

<aura:iteration aura:id="invoiceLineList" items="{!v.invoiceLines}" var="invoiceLine">

  <c:lightningFormElementOutput label="Line ID" value="{!invoiceLine.Name}" />
  <c:lightningFormElementOutput label="Invoice ID" value="{!invoiceLine.Invoice__r.Name}" />
  <c:lightningFormElementOutput label="Description" value="{!invoiceLine.Description__c}" />

</aura:iteration>

The two datasources invoices and invoiceLines are set in the doInit method:

doInit: function (cmp, event, helper) {
    let params = {};
    params.appointmentId = cmp.get("v.recordId");
    helper.callAction(cmp, "c.getInvoices", params, function (invoices) {
        if (invoices) {
            cmp.set("v.invoices", invoices);
            var invoiceLines = [];
            invoices.forEach(i => {
                console.log('i.Invoice_Lines__r');
                console.log(i.Invoice_Lines__r);
                invoiceLines.push(i.Invoice_Lines__r);
            });
            console.log('invoiceLines');
            console.log(invoiceLines);
            cmp.set("v.invoiceLines", invoiceLines);
        }
    });
},

Each of the console.log statements returns expected data.

And notice the H3 with Invoice Lines ({!v.invoiceLines.length}) that outputs a value.

Yet the second <aura:iteration> does not render any values.

1. Why is this happening?
2. How do I fix it?