Trailer brake breakaway failure - possible warranty claim?

Nope, just a mishap that you happen to have caused. whether or not they locked up doesn't matter as to who`s fault it was.

With that said, trailer brakes should fully engage if not lock up. when I first got our current trailer the brakes barley worked. It took some work but found out the brake magnets were weak! replaced the magnets and I could lock up the brakes with 100% braking efficiency. I set my controller so the brakes don`t lock up, but you always want good brakes!