Nail pop means they used (cheaper) nails when they should have used screws. DH says that sometimes they tack the drywall up with nails and then screw too. He said to just pound the nails back in and fill. If its under warranty, the builder should do it. If they are only doing 1 warranty repair visit (weird!), I'd just save them all up, KWIM?

DH says they are likely doing it at 11m b/c you have a 1y warranty.

I read your whole post to DH just now and he says that is poor workmanship. He said it sounds like they used nails exclusively (not just to tack to the studs) instead of screws (which are more expensive and take a bit longer to install, but if you watch Holmes on Homes, you know that that is the right way to do it).