Looks like after woosa got awakening, her immunity to desert illness is gone. i did Nouver a few hours ago and was unpleasantly surprised. can't see anything in patch notes about this
Woosa is not immune to the desert illness. She can however cure the daytime one by using rain skills. These are only in preawakening / succession for her. Did you make sure to use these skills to remove the debuff? Do you maybe have a video of it not working?