Lorentzian magnetoresistance (L-MR) has been widely observed in three-terminal ferromagnet-nonmagnet (FM-NM) tunnel junctions. One possible explanation for this behavior is ensemble dephasing (Hanle effect) of a spin accumulation, potentially offering a powerful approach for characterizing the spin lifetime of emerging spintronics materials. However, discrepancies between the extracted spin parameters with known materials properties has cast doubt on this interpretation for most implementations. Here, we have developed a method to control band alignments in perovskite oxide heterostructures through the use of epitaxial interface dipoles, providing a highly effective method for manipulating the Schottky barrier height and contact resistance. Using these atomically engineered heterojunctions, we are able to tune key parameters relevant to various spin accumulation models, providing an experimental platform which can test their applicability. We find that the observed L-MR is inconsistent with an interpretation of spin accumulation in either the NM material or in interface states. Rather, we consider a mechanism analogous to Coulomb blockade in quantum dots, where spin-dependent tunneling through an ensemble of interfacial defect states is controlled by local and external magnetic fields.