Missing cashier python api example shows alert from beginning to end of video

Hi, I am trying the python api example with the missing cashier video. I am able to run the code successfully but the alert starts from around 1 sec of the video to the end. I opened the client and i can see zone is set and person is detected inside the zone. but somehow it still shows alert of missing cashier. I am using Detector Person Overhead Openvino and also tried other capsules but seems not helping.

any comments are welcome.

many thanks.
Kelvin

Please expand the alert region to cover the detect block.