It's not a glitch, it's a new 'feature'. MS has decided that anyone who uses OneDrive want to collaborate on everything, so attachments of files stored on OneDrive are now links to the files.
There has been predictable outrage on MS Community and User Voice forums, so expect this 'feature' to be rolled back soon, and possibly an option to set a default for attachments. In the meantime, you can right-click on the file name and select 'Attach as copy' from the menu.