While your example gives a quite strange result (but a result that can be explained), I don’t see any overflow value in the rendered image.
[2] = '_portrait2':
size = (800,800,1,3) [7500 Kio of floats].
data = (128.395,129.372,133.618,136.186,136.186,133.618,133.618,133.618,136.186,133.618,134.105,126.99,(...),104.451,99.8682,99.8682,99.8682,106.592,99.8682,100.175,100.175,100.175,100.175,100.175,96.6649).
min = 0, max = 221.713, mean = 95.2109, std = 49.4719, coords_min = (403,36,0,1), coords_max = (636,425,0,0).