Wow, interesting tid bit on the corn in the summer. I wonder how that happens biologically.
Mostly in people's imagination though it is repeated a lot on this forum. I think the general theory is that since corn is fairly high in oil you should feed them corn before they go to bed in really cold winter in winter so they can heat themselves as they digest it. Somebody saw that recommendation and determined that feeding them corn in summer was bad. You get that kind of reasoning a lot on here. Once it gets posted others take it as a given. So, yeah that was probably something read on this forum.
If you look at the nutrients analysis you could get the same effect with high oil or fat mealworms or BOSS, winter or summer. If you look at the ingredients list on the chicken feeds many include corn, which is fed year around. Many of us do not feed extra stuff like this in cold weather and, like the wild birds that overwinter where you are, the chickens make it through the night fine. As long as you don't overdo it this will not hurt anything, any more than wild birds filling up at a bird feeder. Not all wild birds have access to a bird feeder before they go to bed.
Commercial chicken feed from a standard source has all the nutrients chickens need for a balanced diet if that is all they eat. A lot of us like to feed extra that we call treats. As you may have noticed "treat" can mean different things to different people. If you control everything that they eat a rule of thumb is that all treats should constitute no more than 10% of their diet so their diet stays balanced, whether it is a good treat or bad treat nutritionally. The attempt is to be balanced in all ingredients. Scratch is not a balanced chicken feed, it is a treat.
I don't follow that rule of thumb. Mine forage for a fair amount of their feed so I'm not able to micromanage their nutrient intake if I wanted to. In season mine may get a lot more than 10% of their daily diet from some treat. That can be garden excess (especially when I'm cleaning up) or when I'm canning they can get an overload. Orchards are dropping a lot of fruit or berries they can get to may have a lot ripe. It's not every day they get the same excess, it kind of balances out. Studies have shown they can do a decent job of balancing it themselves if they have quality forage so they can choose. I'm a lot more laid back on balancing their diet than some people.
I don't know what to suggest to you. There are people on here that can do an excellent job in helping you micromanage everything they eat if you want to go that direction. I'm not one of them.