Pdfbox form field classes read the default appearance into a member variable early in their life-cycle and don't follow-up to changes in the form field dictionary they are based on. Thus, when creating the appearance stream during pdfFormField.setValue("Hello")
, the former DA value is used.
After setting the default appearance, therefore, you have to instantiate the form field object anew. Then set the field value using this new object.
For sample code look at this answer to How to set the text of a PDTextbox to a color?; here the existing DA value of a text field is changed to contain a color setting operation before the field value is set.