If this is involving Microsoft Exchange Server, then check with whoever is maintaining the Exchange Server mail server around its configuration settings, and around any anti-malware or other mechanisms that might be running on the Exchange Server or on the Windows client that’s running Outlook.
There’ve occasionally been issues with Outlook client and chunks showing up in unexpected ways, as well...
https://social.technet.microsoft.com/forums/office/en-US/bbee3a53-e232-47ae-a3b1-6c81cf4b0955/outlook-stripping-forwarded-attachments-altered-mime
https://social.technet.microsoft.com/Forums/en-US/1591bb59-d3b0-405b-8042-49bd1915558c/outlook-2016-office-365-unable-to-preview-html-files-on-email-attachments
https://www.extendoffice.com/documents/outlook/1830-outlook-prevent-inline-attachments.html
Assuming that the mail message is arriving “normally” at the mail server (that’s only really visible to the folks managing the mail server, though) and if this is only the Outlook client, and if this is not happening elsewhere, then patch to current, upgrade to the current Outlook and to thr current Exchange Server (if that’s involved here) and then check with the folks at Microsoft Support.
Search in the Microsoft forums, as well. This probably won’t be the first case of this misbehavior, after all.
El Capitan is old, and you’ll want to upgrade to at least High Sierra, and test forwarding email with that.