I had this same problem on my 05 wrx some years ago. The spring in the wastegate actuator broke or bent causing the WGA to open under far less boost signal than stock. I got a new one and the issue went away.
If the car was stage 2 before, they probably set a good amount of preload on the WGA which could cause gradual damage to the spring or perhaps they adjusted the arm too far in the other direction when they put it back to stock.
Just tug on that little arm with one hand, it shouldn't move without really having to pull. If you can pull the arm with one hand and a couple fingers then you just need a new WGA or just need to adjust the preload