Text font and size in footer of a part are fixed (to those of score). Changes have no effect.
In a part, the Font face and size options in Style => Header, Footer => Footer Text have no effect. No matter which font or size, it seems remain at the default value: FreeSerif and 8pt, or the corresponding values in the score. The Ok or Apply to all parts buttons make no difference.
In other word, if we want to change the font and size of the footer of a part, we have to do from the score.