I've seen several boats of different manufacturers become weak and begin to fail just ahead of the main beam.

I think it comes from years of the loads just ahead of the main beam, flying ah hull. Then, a semi violent pitchpole adds even more stress to the area just ahead of the main beam.

My Prindle 18-2 began to fail just ahead of the main beam as well. I've seen others.

Good luck, sounds like you'll get it fixed just fine.