There’s a bug that puts random emails from Gmail as all-day events in Calendar. It is unknown how widespread the problem is and Google has not yet responded to the reports.
Source: The Verge
Notifications about changes to Epic’s privacy policy and digital receipts, among others, appear as all-day events in Google Calendar, report 9to5Mac and The Verge. Users have been reporting the random appointments since Friday. It is unknown how the bug causes some emails to become an event in users’ calendars.
It seems to mainly affect users who use the Google Calendar app; it is unknown if it also affects users on the web. In any case, it concerns the apps on iOS and Android. A workaround is to disable the option to automatically convert events from emails into events in Calendar. This can be done via the settings of the app.