debug the following snippet of code and inspect the values as shown in the picture.
        Font font1 = new Font("Tahoma", 0);
        Font font2 = Font.font("Tahoma", 0);

note that font2.nativeFont.fontSize == 1

that happens because of this line in PrismFontLoader#font(family, weight, posture, size)
float fontSize = size <= 0 ? 1 : size;

Is this line of code there by accident how it really means what it is doing ?

Note that fontSize < 0 means use default, but that is all taken care in Font.