I can't find that setting under Advanced in ESET Endpoint Antivirus 6.6. I noticed that not all of my users with Outlook 2016 experience the issue until they get their latest updates via Office 365 subscription. My affected build for Outlook is Version 1801 (Build 9001.2138 Click-To-Run) Monthly Channel.
We do have signatures in our email bodies with a .png graphic in them. Those do come through as attachments, just no text in the body to the email.
I also just realized that it seems to be my Windows 7 users and not the Windows 10 users who are effected by this.