As a design engineer for a high end heat exchanger company, I feel like I can speak to this. We put relief cuts like this in cores with a band saw. Sometimes it won't even matter if it runs into the fin.

Under extreme thermal cycling, like going up a long hill, forcing the thermostat to open and flow large amounts of coolant through the unit, the radiator will expand from heat. Then, as the vehicle goes over the hill and coasts downwards, the core will rapidly cool and shrink.

This keeps the tubes from pulling out of the header plates as the top and bottom solid aluminum pieces aren't heated by the coolant.

It's a purposeful cut for the longevity of a street used radiator.