Be sure to check your theme directories for any mis-use of Magento_Customer/js/customer-data
I have found on multiple occasions when debugging that many devs opt to manipulate customer data to achieve things. Here was one I found recently in the placeOrder: function () {
method of a third-party payment gateway integration that had been internally manipulated within the theme:
var clearData = {
'selectedShippingAddress': null,
'shippingAddressFromData': null,
'newCustomerShippingAddress': null,
'selectedShippingRate': null,
'selectedPaymentMethod': null,
'selectedBillingAddress': null,
'billingAddressFromData': null,
'newCustomerBillingAddress': null
};
customerData.set('checkout-data', clearData);
So search app/design/frontend
for Magento_Customer/js/customer-data
and see what naughties pop-up!