This could be because you have an overwrite of onepage.phtml
in one of your themes or modules.
I noticed in one of our upgrades, which was experiencing the same issue, that #checkout-loader
div was missing the data-mage-init
attribute.
This what the div attributes should look like:
<div id="checkout-loader" data-role="checkout-loader" class="loading-mask" data-mage-init='{"checkoutLoader": {}}'>
If you have overwritten onepage.phtml try this fix and use vendor/magento/module-checkout/view/frontend/templates/onepage.phtml
as a reference for any other discrepancies.