Rear defroster only works when the car is warm

I recently purchased a 2020 Hyundai Venue SEL. I took delivery on December 18th. About a week and a half later the weather got really cold and when I turned on the rear defroster it wouldn't come on. Once the car got warm the defroster came on. The next day we had the same issue. The defroster only comes on when the interior of the car warms up. Not very helpful. I took it to the dealership and they noticed some breaks in the grid wires and fixed them. This did not solve the problem. Then they replaced the switch for the rear defroster. That did not solve the problem. The engineer's suggested that we replace the back windshield. We did that, and again that did not solve the problem. So here we are, the car is back at the dealership and they're about to replace the junction box, they're just waiting on a part. Their Engineers seem to be stumped as to what the problem is. Has anyone else had this issue? Can anyone shed any light on this? Could it possibly be a computer issue rather than mechanical or electrical? It might be worth noting that the day the car was delivered the battery died and it had to be replaced. Don't know if that's related but figured I'd throw it out there. The car's been in the shop longer than it's been in my possession since I bought it. I'm becoming increasingly frustrated and would appreciate any help anyone can give. Thanks in advance.