Outlook Addin - ItemProperties.Add change the mail format

I am trying to add an Item Property as follow:

Microsoft.Office.Interop.Outlook.ItemProperty property = currentMail.ItemProperties.Add("isCopy", Microsoft.Office.Interop.Outlook.OlUserPropertyType.olYesNo);
property.Value = false;

The issue is that it is changing the email format.

Before adding this property the email was sent as follow:

Formatted

After adding this property the email are sent as follow:

No Formatted and with attached file

Furthermore, I have already tried UserProperties.Add. The behavior is the same.