Cannot get default value to work for a select component

I have a select component in a form, which uses a JavaScript transformer as its data source. The transformer returns an array of objects, each of which contains a "currency" and "funding_amount" key. The transformer is triggered anytime a different row is selected in a table.

I have set up mapped options and a default value for the select component, which works just fine. However, the moment I change the "form data key" of the component from an empty value to a string ("funding_amount", which corresponds to the name of a DB column), the default value completely stops working: the select component is blank no matter which table row the user clicks on, and the console throws an error "Cannot read properties of null (reading 'currency')".

Again, this only happens if the "form data key" is set to a non-empty value. Any thoughts on why this might be? Thank you!