似乎reducer函数既尝试使用Immer（由redux-toolkit自动应用）修改代理state，又返回一个新值，这导致了错误。
有关使用redux-toolkit reducer with immer的详细信息
也许可以考虑始终修改缩减器中的代理state：

// Only works in redux-toolkit with immer

removeIndexCart: (state, action) => {
  const IteamIndex_dec = state.cart.findIndex(
    (iteam) => iteam.id === action.payload.id
  );
  if (state.cart[IteamIndex_dec].qnty > 1) {
    state.cart[IteamIndex_dec].qnty -= 1;
  } else if (state.cart[IteamIndex_dec].qnty === 1) {
    state.cart = state.cart.filter((el) => el.id !== action.payload);
  }
};

或者，尝试始终返回新值而不修改代理state：

removeIndexCart: (state, action) => {
  const IteamIndex_dec = state.cart.findIndex(
    (iteam) => iteam.id === action.payload.id
  );
  if (state.cart[IteamIndex_dec].qnty > 1) {
    const updatedItem = {
      ...state.cart[IteamIndex_dec],
      qnty: state.cart[IteamIndex_dec].qnty - 1,
    };

    const updatedCart = [...state.cart];
    updatedCart[IteamIndex_dec] = updatedItem;

    return {
      ...state,
      cart: updatedCart,
    };
  } else if (state.cart[IteamIndex_dec].qnty === 1) {
    const data = state.cart.filter((el) => el.id !== action.payload);

    return {
      ...state,
      cart: data,
    };
  }
};