Purple dots after LCD change

I see these happen at work occasionally and none of us know what really causes them to happen, sometimes they show up right away, sometimes they show up later. For us they appear to look like a purple "liquid" almost in the corners of the display rather than dots like that. The display was probably just a bad egg. I'd have it exchanged and tell them it was defective. I would also explain to your customer that you had received a defective LCD and that you'd have it replaced, and personally I wouldn't charge them for it as it's not their fault. And swapping a screen on one of them doesn't take that long anyways.

If/when you do that and have a new one, just for the sake of being careful I would make sure you don't have any glass particles, or anything of the sort laying in the area that the screen lays in. And on the board I would take some isopropyl alcohol and a brush and make sure that the contacts on the connector on the board and the screen are clean. That won't fix the LCD you have now, but it may prevent some issues when putting the new one on.