Gray bars flickering after repair

The customer reported that the bars were flickering at the bottom of the screen.

Then if this is the second one you do that has similar or identical symptoms, then I suspect you have received a bad batch of screens.

I thought that would be it too but the thing is after the first time it happened, I switched suppliers.

There's a lot of crap out there...are you paying top dollar for your parts or buying the lowest cost stuff? Otherwise, if you are confident that you have good parts, then you are either unlucky or you are doing something wrong. If you are inexperienced, then look through the iFixit guides to see if you are forgetting anything or doing something wrong.

I should add that you don't need to pay top-dollar. Expensive and good quality can be mutually exclusive. What I wanted to say is that, as a repair shop, you should look for the best quality parts you can find at a price point where you can be both competitive and profitable so that you don't have customers coming back because of flaky repairs. Otherwise the cost of eating warranty repairs will quickly out pace your profits. - Good luck!