Actual Image not shown in the Body of Apple Mail client Products witnessed: iPad 2, iPad &amp; iPhone X; iOS 11+;

We send email broadcast of our reports to corporate customers; The email carries an attachment (*.pdf) as well as an image embedded in the body (any image *.jpeg, *.png, *.jpg etc; We use the Content-ID {CID} to attach the Image).

We witnessed that some iPhone X and iPad users were complaining that the image embedded in the body is an old one from a previous mail and not the actual one that was attached. We observed that these users were using Apple's native email client. We had checked all other email clients' and the image embedded into the body was the actual one; Email clients' under our observation were MS Office Outlook 2016, Outlook for Android, Outlook Web client, Samsung Mail Client for Android, Hotmail, Outlook and Yahoo web email interface all displayed the actual image except for Gmail, where the image is not shown at all for some security reason.

We would have been happy if the Apple's native email client did not show the image at all, but having shown some older image from a cache source in its device have sparkled lot of criticism to us.

Did anybody came through this problem? How did you all overcome this?
Are your CIDs universally unique? I.e., do they change from email to email? RFC 2392 requires this (https://tools.ietf.org/html/rfc2392). Although it mentions that most reasonable implementations would not index content separately from the message itself, which is (one of the reasons) why it usually isn't an issue.