This API call is suppose to return all funding instruments, however it only returns one. Also there is no field in the funding instrument that would indicate it as being the default. This is shown on Twitter’s Ads UI.
We also tried it on twurl, same result.
This is what we are using on twurl:
twurl -H ads-api.twitter.com -X GET "/1/accounts/18ce53v046c/funding_instruments?with_deleted=true"