In this work, we explain the phenomena of ferroelectricity modulation in the ferroelectric (FE) layer due to the residing dielectric (DE) phase. This work also demonstrates the impact of random spatial fluctuation of DE phase for multidomain Metal-Ferro-Insulator-Metal (MFIM) capacitor and Negative Capacitance (NC) FDSOI FET. Using well-calibrated TCAD models, we found: 1) the presence of DE phase causes non-uniformity in the polarization (PFE) and potential (VFE) inside the FE-layer; 2) non-uniformity in the PFE modulates the average FE capacitance (CFE) due to an increased FE domain interaction; 3) the increase in DE phases decreases the CFE; 4) higher FE thickness (TFE) with higher DE phase exacerbates the CFE variation, thus, in NC region the PFE becomes direction-dependent; 5) larger DE grain sizes show higher bias directional dependency in the NC region; 6) variation of the coercive field (EC) within the FE layer with DE phase, predominantly increases the bias directional dependency in the NC region and thereby introduces the higher reliability concerns for NC-FETs.