Ask for representative stacks through the intended transfer into the actual locating tray, with the proposed separation method, support arrangement and single-panel confirmation defined. A seal is not a thickness count, and I wouldn't pick a detection method without the panel and surface details.
Then the next trial needs a different scope, not simply more repetitions of that seal test. Have the supplier explain how a double or uncertain pickup is detected and handled, and what evidence shows the delivered panel remains suitable for the tray.