I’ve had similar experiences. I built a PML Patriot for my L1 cert, and used the 30 minute epoxy. It used Quantum Tube and 1/16″ G-10, both pretty flexible. I also had problems with cracked fillets, even a chunk came off once so I had to re-do one of the fillets. The hobby epoxy seems pretty brittle. I was never able to determine whether it was fin flutter on the way up or a hard landing on the way down. I guess you could try the upgraded parachute idea now that the rocket is already built.
Since then I’ve used Aeropoxy and Colloidal Silica on my subsequent builds. I’m impressed with it so far, no issues yet.