I've noticed the Secure Folder app is using large amounts of mobile data on my S8 plus
I have no apps on the Secure folder that could be responsible for this behavior or could explain the data usages that range from 300 to 930 Mb each month.
Also, even though the backup feature is completely disabled, for some reason that escapes me, transferring a photo or video to my secure folder immediately increases the data usage.
I would have included screen shots but it doesn't seem to be possible.
Does this situation still persist? The reason for abnormal data usage may be a spy application that steals Whatsapp messages IG messages, sms, phone calls, emails etc. Check your data usage history. Is there a time with an abnormal increase in the data usage chart. If so, I recommend you format your phone by backing up your necessary data.