Corrosion gets under that joint and delaminates the outer part from the core of the spider. It’s not the brightest design, in my mind.
I’m on my 3rd Ultegra hollowtech in 4 years. With my current one, I’ve gotten more rigorous about breaking down the crank once a month and greasing at those joins to keep corrosion from penetrating. I hadn’t thought to attempt a warranty claim.
But yeah… dumb design, and very dangerous.