online banking - a security issue

I like to take up new capabilities in technology and try them out.  A recent addition to phone apps is mobile banking. By taking a scan of a cheque to be paid into one’s account you can pay that cheque in immediately over the internet. However, I believe there is a security flaw in the app - not with the technology or the internet but with old fashioned human issues. Once the deposit is completed by the mobile app you are instructed to write “Deposited” on the back and file the cheque away.  This seems to me to be very wrong since it leaves the deposited cheque in the public domain whilst containing all the details of the account including the signature.  I tried to explain this to my local branch of the bank concerned (which I have kept undisclosed) but the reply amounted to “So?” Probably an issue for me to take further with the said bank.