The read status of an Exchange Online mailbox is not synced between endpoints
You are logging in to an Exchange Online licensed mailbox from multiple endpoints, including Exchange Online Active Sync mobile clients. In some cases, when a user sets an email message as Read, the read status is not reflected/synced over to all other mailbox endpoints (Outlook client, OWA or Active Sync).
Solution
Section titled “Solution”This is a known software bug in Exchange Online, as per https://docs.microsoft.com/en-us/exchange/troubleshoot/user-and-shared-mailboxes/email-message-read-status-not-update. The bug will be permanently fixed in an coming Exchange Online software release. You can either follow step by step instructions in the above article to provide a temporary workaround to the issue or you can try converting your licensed mailbox into a shared mailbox. Shared mailbox end-user usage instructions can be found at: https://support.microsoft.com/en-us/office/open-and-use-a-shared-mailbox-in-outlook-d94a8e9e-21f1-4240-808b-de9c9c088afd?wt.mc_id=365admincsh&ui=en-us&rs=en-us&ad=us.