Need help debugging frontend JavaScript on the checkout page: my checkout fields are showing validation errors as soon as the page loads, before the user even types into them

I’m on Magento 2.3.5-p1 and I’m losing my mind trying to debug an issue I’m having on the checkout page.

As soon as the checkout loads, all of the mandatory fields such as email address, first/last name, etc. are immediately highlighted in red and displayed as if there is an error, like this.

This happens even before the user focuses on the fields, types anything into them, or interacts with them in any way.

My usual methods of JavaScript debugging don’t seem to work when it comes to Magento, because everything is loaded dynamically via KnockoutJS or AJAX, and all the code is heavily abstracted. It doesn’t help that there are two dozen events attached to the fields so crawling through the code would take days. So I’m not sure where I should even begin looking.

I’m using two custom modules: OneStepCheckout and PostCodeAnywhere, however I’ve tried disabling both of these and this does not resolve the issue.

Can anyone suggest an approach to solving this, please, or at least suggest a method of straightforward debugging I can use with Magento’s JavaScript?

Thank you.