Problem
"As a delegate of a calendar, when updating an existing meeting, I'm unable to delete or change previous attachments. The attachments appear deleted, but once I re-open the meeting or send the new invite using the delegator's account, attachments reappear."
Solution
This is a known Outlook web and desktop issue and has been reported to Microsoft. As a workaround:
Create a new meeting.
Copy and paste the relevant details from the old to the new meeting. Then, attach the updated files to the new meeting.
Cancel the old meeting, and send the new meeting invite.