just because boost is constant doesnt mean that it is. boost being taken from the manifold doent mean thats what the turbo is doing. if there is a leak then the turbo needs to actually run for example 20psi to try and make 15psi at the manifold because of a post turbo pre manifold leak. same could be happening with an exhaust leak. the ECU is telling the turbo to run more because its not getting to peak boost when it should be.
installing an UP should not of done anything wrong. is your butt dyno off? if thats all you installed and you notice a difference then you installed something wrong. go back and double check your double checks.