Hello @wernstrom,
This does seem like odd behavior.
Do you happen to be using a custom checkout/third party checkout provider or BigCommerce’s express checkout?
Since this method of adding users from the checkout page is meant to sync subscribers via the BigCommerce integration, if you were using a custom/third party checkout provider, I would recommend reaching out to them for further details on how to push these subscribers through to BigCommerce and then to Klaviyo.
Alternatively, if you were using BigCommerce’s express checkout option, instead of the checkout.html, I would also recommend double checking the checkout-express.html file. If that file also seems empty of code as well, I would recommend reaching out to BigCommerce’s support for further details or reaching out to your developer for more information as to how your checkout page is coded as these pages typically will have more code pertaining to your checkout process than just one line.
I hope this helps!
David