I've had this problem for the entire time I've owned my 2018 G80 sport here in Hot Texas. It has been to the shop multiple times for it. Same symptoms everybody describes. For me, 45+ minutes driving down the freeway, AC stops blowing cold air. I can tell it's happening because with the system set on Auto and about 72 degrees, everything is fine for the first 45 minutes or so. Then my wife and I look at each other about the same time and say "seems like it's getting pretty warm in here". Reach down feel the air coming out and it's not. Or very weak, and not cold. No amount of changing switch positions/settings makes a difference for us. Tried everything. Had it in the shop multiple times. At first Genesis dealer claimed ignorance of this issue. They changed cabin filters, spit on it, banged things with a mallet or whatever else they do when they have no idea.
Then after a couple of years, they said "oh yeah" Genesis figured it out. Said it is a bad thermistor. And they replaced it. No Go. Same problem. It's not every time. Only on the hottest of days when I still have a long way to go.
What I have figured out is I can turn off the A/C system completely, roll the windows down. Wait 15 minutes, and turn it back on and it works fine. Sometimes for several hours. I've tried leaving it off for lessor times, but anything less than 10 minutes certainly doesn't work for me. Sometimes as early as 12/13 mintutes is enough. But I've found 15 always works.
Stupid Genesis.