Doesn't it seem like Shizune heals more frequently after her second heal? Like after her first heal we knocked her down to 2 hp. And then she healed second time well afterwards we couldn't even get close to getting her back down because she kept healing.
If you have no chases until the next round, she heals, poisons, repeat. It depends on who else can move. For me, she always heals after Hinata because that's the last chase left until next round.