Read/Unread Discrepancy Between Outlook for Mac and Outlook for iOS

My colleague uses Outlook 2016 for Mac and the Outlook app on iOS. Pretty sure he’s on the latest public builds (Mac High Sierra, he’s not on Office Insider or using any iOS beta or TestFlight builds). He’s reported that within the last couple of weeks, his read messages on desktop routinely appear as unread on mobile. This doesn’t necessarily affect the unread notification count on the app’s badge but rather within the app, where messages still appear as unread with a dot beside them. According to him, they may still appear unread for days if he doesn’t manually mark them as read on mobile. Anyone have insight into what might be happening here? Any troubleshooting steps I might suggest to him? Thanks!