Daughter denied access to smart saver account opened by parents.

My husband and I opened a Smart Saver account for my daughter 17 years ago. Any cash received for birthdays or Christmas has been saved in the account. This weekend I went to pay a cheque in to be informed that her name is no longer on the account! It is shown on the passbook but for some reason not on the computer system, so I could not pay in a cheque made out to her by name. How could this be possible? All the correct documents were presented when we opened it - birth certificate, passports etc. How can the main account holder be removed from the account? The best my local branch can offer is that we close the account and open a new one in her name! Fortunately as trustee’s we can still do this but if we had left it until she was 18 then who knows what problems she would face. I’m furious and in light of GDPR, this feels like fraud in reverse! Anyone able to advise what I should expect from building society manager later this week? Do I have a legitimate complaint? It certainly feels like it. Is it worth making a fuss about? Thanks