Symptoms
In an Exchange Server hybrid environment with both on-premises and Exchange Online (EXO) mailboxes and Microsoft Graph API enabled, on-premises mailbox users cannot view free/busy information for EXO mailboxes in classic Outlook. Instead of correct free/busy information, users see only slots that have time mentioned. Free/busy information displays correctly if the administrator disables the Graph API and reverts to using Exchange Web Services – (EWS).
Resolution
To fix this issue, install the following security update or later updates: