For IMAP mail accounts these things are shared across devices. In those weird business situations where multiple folk share an account it is also information shared to all.

So is it possible that these mails have been rplied to on another device or user? Right click on one of the message and select open and then "in conversation" is the message only shoown, or a reply as well? Right click the folder, select properties and then the reapir button. DO the arrows just leave?