[Python] OpenCV drawContours fills "inner" contours - why?

I use OpenCV findContours() to extract contours, which I then draw in a figure. The contours themselves sometimes enclose holes. When using drawContours(), these holes are not drawn, but filled instead. I am using the full hierarchy, but do I also need to pass that hierarchy into the drawContours function somehow?

import cv2
import numpy as np
import matplotlib.pyplot as plt

f = "file"
width = 853
height = 418

img = cv2.imread(f,-1)

roi = np.zeros(img.shape[:2], np.uint16)
roi = cv2.rectangle(roi, (13,78), (width,height), 65535, cv2.FILLED)
mask = cv2.bitwise_and(roi, img)

threshold = 350

ret, th1 = cv2.threshold(mask,threshold,255,cv2.THRESH_BINARY)

contour,hier = cv2.findContours(th1.astype(np.uint8),cv2.RETR_TREE,cv2.CHAIN_APPROX_NONE)
canvas = np.zeros(img.shape[:2], np.uint8)
canvas2 = np.zeros(img.shape[:2], np.uint8)

for cnt in contour:
cv2.drawContours(canvas,[cnt],0,255,2)
cv2.drawContours(canvas2,[cnt],0,255,-1)

fig, ax = plt.subplots(3,1)
ax[0].imshow(img, 'gray')
ax[1].imshow(canvas, 'gray')
ax[2].imshow(canvas2, 'gray')


Which produces the following results

How can I prevent the inner contours from being filled?

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