Mulberries and several other fruit species that generally love heat, leaf out and flower late, and are low chill (figs, grapes, persimmon, etc) are more sensitive to late freezes than many stone fruits. A mild frost that won’t hurt many stone fruits will defoliate mulberries and the others mentioned. I don’t know if the fruit is more sensitive but the new foliage certainly is. The fruit tends to come late on things like mulberry so it is the foliage and new shoots that shows damage when late freezes happen.