When you use Outlook for Office 365, Outlook 2019, or Outlook 2016 to attach an Outlook item to an Office 365 Modern Group Calendar appointment or meeting, the attachment is not retained.
This issue occurs when using the Outlook Item button on the Insert tab of the ribbon. This is a known limitation in Outlook with Office 365 group calendars.
Copy and paste the contents of the desired Outlook item into the body of the meeting or appointment. Alternatively, you can upload the Outlook item to Files in the Office 365 Group or to OneDrive for Business and then include a link in the body of the meeting or appointment.