im flowering white widow from nirvana seed co right next to a sweet tooth and i have noticed that the white widow's big fan leaves started to turn yellow about 2-3 weeks into flowering. i think its just a genetic trait of the ww because the sweet tooth isnt yellowing at all. the same leaf yellowing during flowering happened with a northern lights cross i harvested.
personally i wouldnt worry about it .once they're yellow enough to come off easily i pluck em off. this also exposes the buds that are being shaded to direct light.