What about Cte issues? The coefficient of thermal expansion will be fairly similar for both parts I assume, since they are both more or less are aluminium. Motor casing heats up during flight, probably expanding a significant amount. The fins are in the airflow getting a good dose of convective cooling off the not insignificant airflow. They are also in the lew of the shockwaves largely, so a high total temperature of a normal shock wave probably is significantly cooler in the post-shock area where most of the fin surface area is. So that is two things pointing to significant temperature differences, and change rates, during the flight.
Also consider the epoxy between the surfaces as an insulator. There will be only a low heat flux through that and only for a short time. Sounds like a potential differential expansion issue, with some other not insignificant forces at play.
Let me know if I got any of that aerodynamics stuff wrong. I am an electronic engineer