Voltage regulation in power distribution networks has been increasingly challenged by the integration of volatile and intermittent distributed energy resources (DERs). These resources can also provide limited reactive power support that can be used to optimize the network-wide voltage. A decentralized voltage control scheme based on the gradient-projection (GP) method is adopted to minimize a voltage mismatch error objective under limited reactive power. Coupled with the power network flow, the local voltage directly provides the instantaneous gradient information. This paper aims to quantify the performance of this decentralized GP-based voltage control under dynamic system operating conditions modeled by an autoregressive process. Our analysis offers the tracking error bound on the instantaneous solution to the transient optimizer. Under stochastic processes that have bounded iterative changes, the results can be extended to general constrained dynamic optimization problems with smooth strongly convex objective functions. Numerical tests using a 21-bus network have been performed to validate our analytical results.