If you see an IBAN partially masked returned, (for example BE299670xxxxxxxx), that's just a placeholder.
The reason for this is because the user actually doesn’t have an IBAN assigned to their EUR account just yet. In Wise, customers can open currency accounts without any attached bank details.
Solution
What to do next?
In this particular case, you can advise the user to log in to their Wise account and simply request unique bank details (IBAN) for their account. Once they do, that API will start reflecting this too.