Using New Outlook on Windows for personal email, calendar, and contact management
This could be a current limitation or regression in the new Outlook rather than something wrong with the calendar event itself.
As a quick workaround, try opening the calendar event first and use Insert > Attach File or drag the file directly into the attachment area after the event window is open.
For an email, you can also try using More actions (...) > Forward as attachment if that option is available, then attach it to the event.
Since this worked previously and has now stopped, I’d also check whether the new Outlook app is fully updated. If the issue happens consistently with both emails and regular files, it would be worth reporting it through Help > Feedback with the Outlook version/build and a short recording of the drag-and-drop behavior.
If anyone else is seeing the same thing after a recent update, that would also help confirm whether this is a wider regression.