This is always for a single record.
I've tried dragging the formula to the report form and setting its "CanGrow" property to True.
I have also tried adding the TextObject back on the form and inserting the formula to the TextField instead and setting the CanGrow property of the TextObject to True.
In both cases nothing populates the letter. Therefore, I think I'm going to have to fall back to creating a duplicate of the letter and choosing between them in code based on the store.
Thank you though for all the suggestions, as these were practicle.