Thanks for looking into it!!
Unfortunately we are using a deployed instance of killbill even for development, so it will take me a little while to set up a local environment and do proper debugging. I will get back to you once I have everything running locally.
What I can tell you is that we use Stripe payment element, and the process of linking a bank account works fine, it even creates the payment method on stripe, but when trying to refresh the payment methods in killbill it doesnt sync it.
Here is a screenshot of my test customer on Stripe:
And here is what killbill syncs for that customer Kaui