We are seeing a lot of our Mac users who are using the Native Mac App complaining that their iphones don't show emails as read, and looking into it. If you go into webmail you can see there they aren't marked as read.
I've tried logging on to the mac and changing the settings via terminal to mark an email as read as soon as it's looked at. Using, "defaults write com.apple.Mail MarkAsReadDelay 0"
But some of the Mac Clients running different versions of OS X all at least 10.13.6 are seeing this issue.
Macs are the bane of my life and i think are going to force us into abandoning Groupwise for Office 365 which i hate the idea of but is going to happen here due to the fact we don't have a reliable client for Macs.