Computed when composed formulas don't just execute when the document is composed. They execute when the form is edited and the field doesn't exist in the document. So they're really computed-when-field-doesn't exist formulas, which is also true for a new document.
Create another field whose formula deletes the rich text field on saving the document. For the rich text field, use a DbLookup to initialize the field. This gives you, in effect, a computed-for-display rich text field.