Learning - Related Materials not saving on an activity

Summary:

Our learning administrators are attempting to add pdf files to the Related Materials of an activity. The files appear to be imported correctly however when they leave the page and then return to the activity, none of the files appear in the attachments. We've verified that no attachments are visible to the learners.

The same files have been uploaded to other activities previously without any issues.

Can someone please advise?

Content (please ensure you mask any confidential information):
Files are added to the Activity successfully

When the administrator attempts to go back to the Activity, all files have disappeared from the Related Materials