It's not clear exactly what you are doing here. On the surface, this is not normal behavior for a FileMaker file.
Is it possible that the text inside the field is formatted to use Helvetica and the field, in layout mode is specified to be Arial font? Text formatting applied directly to the data in Browse mode will supersede formatting specified for the field object while in layout mode. This often happens if text is pasted from another source into a field or if the user uses the formatting tool bar while in browse mode to format text inside a field.
This formatting can be removed and then the layout's formatting will apply.