In a Microsoft Exchange Server 2016 environment, the mailbox usage status bar in Outlook Web App (OWA) sometimes presents incorrect mailbox usage (almost full). You can check the mailbox usage in OWA by selecting Mail Settings > Options > General > My Account.
To fix this issue, install Cumulative Update 9 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Learn about the terminology that Microsoft uses to describe software updates.