Error while trying to Update an external banking account

While trying to Update (not Create) external banking account using API iby_ext_bankacct_pub.update_ext_bank_acct getting following error,

1: The external bank account already exists

2: One of the assignment of this bank account is related to operating unit (<OU name>).

We are passing the ext_bank_account_id populated in the input parameter record. So obviously its an existing account and we want to update that. Why does it complain it already exist ?

Also, assignment is not what we are trying here, JUST updating a DFF on the iby_ext_bank_account field

It just throws this error and nothing gets updated.