Most likely due to the PCM venting the excess boost from the system to prevent it from being damaged from the change to the wastegate.
Once it gets the boost back down to what the stock wastegate should be maintaining, it levels back off at normal pressures.
The question should be: How much performance are you losing overall by forcing the boost levels out of acceptable parameters with the wastegate spring mechanical mod.
The PCM could use the time spent in neutralizing the effects of your mods to provide more consistent performance, and yield higher overall output...
Tune the PCM; not the engine - Then they will come...........
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