I have also noticed the compass does not work correctly when operating in our pool since the pool has a lot of steel reinforcement. I can calibrate the heading away from the pool but it loses its heading after the first lap.
You are not alone. I have seen that happen also in my Association’s swimming pool.
Supposedly the OROV2.8 IMU’s compass was not used for heading when the motors were on and would only work correctly when the motors were off. When the motors were on the accels and gyros were used for the heading.