Never heard of the place, so I guess that shoots the whole "everybody" bit.
Let think about this. Problem moved with bulb. So as you stated logic shows the problem starts from where the bulb plugs into the harness, and could be anywhere from there forward. I agree.
And if the problem moves with the bulb, why the heck would you want the harness checked?
I deal with China products/suppliers/so called customer service nearly daily.
Poor assembly can happen. Loose connection/poor solder job and when you receive the product it doesn't work. You pack it up, it gets "lovingly" handled back to China.
Tech opens the box pulls out the bulb and plugs it in. Guess what, it works.
Instead of checking any further he marks no problem found and moves on to the next one. Different attitude there, they get paid by the number they can go through in a day, and have to meet quotas, so they don't do any deep testing. Company sees report, emails you that your product is fine, and you now have to pay to get it back. Not their fault you sent them a perfectly good product.
So now you pay to get it back. It get the same "loving" care as it returns. You plug it in and it either works or it doesn't.
And when it stops you are right back where you started.
Two of the main companies I deal with are exact opposites. One has cheap products, as in low prices. Okay maybe a bit more. They also have less then stellar material, quality control, and good luck if you have a problem.
The other, lot higher prices. But better material, better quality control, and I can pick up the phone, or send an email, or do a live chat, and I am speaking to someone who knows the product, knows proper diagnosis, and speaks fluent English.
And one other thing, most of items like your lamps are built in only a couple of places.
They are built to contract specs. And lets just say sometimes a run or two gets built out of spec. They don't toss em, they sell em, and they also copy everything, and build their version out of cheaper material and so on.
This DDM place is nothing but a distributor. They buy a container load of product.
They don't make the stuff, and I doubt they even check it. It get's pulled off a shelf or box, and yep I've seen where multiples of a product are just dumped into a box.
And it gets "lovingly" shipped to you.
Same as the 30 or 50 or 2000 other distributors do with the stuff from the same place in China.