The banking team confirmed that according to PSD2 regulation, banks are obliged to return only payment accounts. As for the non-payment ones (savings, investment, and others depending on the bank, maybe even credit cards), the bank decides whether to return them or not.
Unfortunately, if the banks returned only one account, then there is nothing we can do about it.
The only option would be the user, you, contacting the bank directly and clarifying what types of accounts are being returned via their Open Banking channel.
Thank you for your understanding.