I’d think of two possible explanations here. One might be some bad routine testing, like plugging in a screen with board turned on and working, that would easily short the backlight circuit. The second and more likely possibility would be a faulty testing screen or relevant connecting cable. Faulty magsafe or I/O board can be ruled out in my opinion. It would be better to see how these guys work with the boards after they get them to be sure, but considering how uneasy is to damage always the same components, there must be something wrong with their tools or procedures and I wonder why they’ve not realized that themselves. If I was in your shoes I’d seriously consider stopping supplying repair services unless they start sending the full machine for repair and testing at your end.