Many things are more complex than they seem. In this case, look at the second tab of the field properties. If the Store contents as HTML and MIME is checked, then when the document is saved, the rich text will be converted to HTML and MIME, and when it is opened again, it will be converted back to rich text for display.
The easiest way to tell is to look at an existing document's properties from the view, not from the open document
, and see if the Body field, or whatever your rich text field is, is in Rich Text or in MIME. If it is in MIME, the Body fields will each say MIME Part, as below: