Precise electrostatic control in van der Waals heterostructures is commonly described by a geometric capacitor model, which assumes that the internal electric field is fully determined by external gate voltages. Here we show that this description breaks down in the presence of interfacial charge transfer, where quantum capacitance modifies the internal electrostatics. Using interlayer excitons as local probes, we observe a dipole-dependent energy splitting that persists even when the gate-induced geometric field is nominally canceled. This splitting reflects incomplete cancellation between gate- and carrier-induced fields set by quantum capacitance, while many-body renormalization is largely symmetric with respect to the two opposite dipole orientations, allowing the splitting to provide a direct optical measure of the local chemical potential. Our results reveal the central role of quantum capacitance in interface electrostatics and provide a route to probe it in operando.