It's not a defect. Murphy's Law applies to damn near anything, and although there's a low chance of stuff getting sandwiched between the pads and rotors, it does still happen. If you don't want that to happen, you could either custom-engineer a way to cover the rotor, however, it would cause your vehicle to be unsafe to drive as it would be detrimental to the brake system's ability to shed heat.
If it happens again, and persists, consider taking the debris out yourself (if you know enough about brakes) or take it to a reputable automotive service shop to address the problem.
"Why would I want to profess my feelings for some 3D girl?" - Tomoya Aki from the anime Saekano