I am going to second what "Hysteria" posted about this issue. It is just the nature of LCD displays. They simply do not do well in temperature extremes. In the past, I have had plug and play satellite radio receivers with backlit LCD displays installed in various cars. On hot days, the displays would be dim. On very cold days (below freezing), the display would disappear entirely until the car interior warmed up. Once the radio returned to a "normal" operating temperature everything would be fine.
The display on my '11 GLS radio fades after sitting out on hot days. The first time it happened, I just chalked it up to the normal function of LCD displays. After driving for 20 minutes or so with the A/C on, the display would go back to normal contrast as the car cooled down. It will also not surprise me this Winter on the first 20-degree day if the display disappears entirely or is severely segmented until the car warms up.
One way to attempt to avoid these display issues would be for Hyundai to change the type of glass they use on the LCD display on the radio. It appears the radio uses a different type of glass on the display when compared to the LCD clock in the car. Temperature tolerance of LCD's is a function of both the type of liquid crystal used as well as the type of glass used for the display.
There really is no reason for Hyundai to feel compelled to issue a recall for this issue. In fact, it is surprising they have not issued a TSB to dealers explaining this is normal and to stop replacing radios under warranty only to have the issue not be resolved. It might be annoying, and not be the answer we want to hear, but it is an unfortunate aspect of LCD displays. I would rather hear that from the dealer than have them replace the radio (and give the customer an expectation of problem resolution) only to have the issue not be fixed. That only serves to irritate customers even more (and drive up unncessary warranty claims and reimbursements.)