Old emails remarking themselves as UNread
In Mail on my iPhone 6 a few already read & opened emails will re-mark themselves as un-read, un-opened. This happens a few times a day. I have noticed that these random emails that are remarking themselves had been received and opened by me exactly one week to the day. Does anyone know how to stop this happening? It's driving me crazy. Many thanks for your help.
iPhone 6s Plus, iOS 11.1