About opencv approxPolyDP function. Program can't find rectangle that I want if I change the angle

Question

// I used cvtColor, inRange , bitwise_and, GaussianBlur, Canny functions before.
vector<vector<Point>> contours; // to store contours
vector<Vec4i> hierarchy;

int mode = RETR_CCOMP;
//int module = CHAIN_APPROX_NONE;
//int module = CHAIN_APPROX_SIMPLE;
//int module = CHAIN_APPROX_TC89_L1;
int module = CHAIN_APPROX_TC89_KCOS;

findContours(img_edge, contours, hierarchy, mode, module);

sort(contours.begin(), contours.end(), compareContourAreas);
drawContours(image, contours, -1, Scalar(255, 0, 0), 1);
imshow("ContourImage", image);

vector<Point> contours_approx;

for (int i = 0; i < 5; i++)
{
    double length = arcLength(contours[i], true);
    approxPolyDP(contours[i], contours_approx, 0.1 * length, true);

    if (contours_approx.size() == 4)
    {
        break;
    }
    contours_approx.clear();
}

for (Point i : contours_approx)
{
    cout << i << endl; // to check coordinate of vertex
}

drawContours(contour2, vector<vector<Point>>(1, contours_approx), -1, Scalar(0, 0, 255), 1);

// to find vertex
circle(contour2, Point(x0, y0), 5, Scalar(255, 0, 0), -1, -1, 0); // blue
circle(contour2, Point(x1, y1), 5, Scalar(0, 255, 0), -1, -1, 0); // green
circle(contour2, Point(x2, y2), 5, Scalar(0, 0, 255), -1, -1, 0); // red
circle(contour2, Point(x3, y3), 5, Scalar(255, 255, 255), -1, -1, 0); // white
imshow("contour2", contour2);

In first, second picture, there is a result that I want. There are circles at vertex of rectangle.

But in third, fourth picture,I modified the angle of cuboid, and there is a result that I don't want. Circles were in very small rectangle above the large rectangle.

In first, third picture I guess that findContours function works well. And I guess that there is a problem in approxPolyDP function.

To locate circle in the largest rectangle, what should I have to do? Thanks for your answer