Mayby this is a bug -
at one uncertain moment both mages in party lose thier bonus spells from perk "additional spell 1st level" (or something like that - i see name on another language),
i can't give you "Steps to reproduce ..." cose i dont know them, but bonus spells appears after taking this perk and disappears later - i mean after resting mages have 4 spells to use instead of 5,
hope this may help to make game better.