Issue with cascade classifier

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I am using detectMultiScale to process two images A and B. When I process A alone, there are no detections, however when I process A after B there is one detection.

CascadeClassifier classifier("classifiers/haarcascade_frontalface_alt.xml");

Mat imageA, imageB;
vector<Rect> face_detectionsA_beforeB, face_detectionsA_afterB, face_detectionsB;

string fileA = "A.jpg";
string fileB = "B.jpg";

imageA = imread(fileA, 1);
imageB = imread(fileB, 1);

classifier.detectMultiScale(imageA, face_detectionsA_beforeB, 1.2, 1, CASCADE_FIND_BIGGEST_OBJECT, Size(64, 64)); // 0 faces detected
classifier.detectMultiScale(imageB, face_detectionsB, 1.2, 1, CASCADE_FIND_BIGGEST_OBJECT, Size(64, 64)); // 2 faces detected
classifier.detectMultiScale(imageA, face_detectionsA_afterB, 1.2, 1, CASCADE_FIND_BIGGEST_OBJECT, Size(64, 64)); // 1 face detected

It is like the output of the current image depended on the output of the previous one, which doesn't make much sense to me... I don't have this problem (i.e., I don't get any detection when I process A after B) if I replace image B by other images, or if I load the classifier every time I use the detectMultiScale function.

Any ideas about what could be happening?