hmm, now you got me, the spring in the actuator might be snapped?
Umm, normally, you should bareeely be able to pull the arm downward out of the actuator, it should feel real stiff, this is the tension holding the wastegate closed until boost builds up (then when the boost pressure on the actuator diaphragm is equal to the spring tension, the wastegate will open a crack and bypass enough to maintain that target boost)
Are you able to move the actuator arm freely or with little effort? if its all stiff and working properly, the wastegate 'flap' might be getting physically stuck open in the back when boosted hard and it opens maximum, this is unusual, do you have a new or modded downpipe?
This may not apply to you but sometimes aftermarket downpipes have a little plate that inserts into the back of the turbo to split the wastegate gas flow from the turbine flow, and a wastegate could get stuck on this plate if clearances are not all checked out at installation.
When doing all this inspecting make sure the hoses between your actuator, compressor and whatever factory boost control are all in good condition, any split leads to overboost which you may not pick up on without a boost gauge.