They don't sometimes really show the increasing amount of buffing so it is hard to say. But for Hinata, she will passively increased her defense when her hp is low, so then her hp is fully restored, she won't be as tanky as how she was in the lethal situation. Only Kimimaru or some other ninjas with curse marked passive can stack the buff
For your Kureina's skill, this is a default thing. For healing jutsu, the amount of life recover is based on the Ninjutsu stat, her shield is based on half of her resistance stat. Most of the shield creating moves only generate shield based on 1/2 resistance but Earth main's Mystery is Super stone fist jutsu ( give shield with full resistance attribute).

Like Naruto's or Hinata's or Anko's Passive? Their buff is temporary, not stacking up like Kimimaru so it will disappear when they have healed back.