The emergence of magnetism via a proximity effect has been exploited in many low-dimensional materials, including ultrathin film and 2D materials. By doing so, one can realize magnetic properties in these materials not observed in the bulk nor achievable via doping or functionalization. Magnetic proximity effects (MPEs) have been studied in a variety of magnetic heterostructures for many applications including spintronics, valleytronics, and topological phenomena. Crucial to the observation of a theoretically predicted MPE is an abrupt interface with little to no interdiffusion or interface roughness. Nonidealities at the interface can give rise to behaviors that appear to be a MPE. This study addressed the constraints on interpreting evidence for MP in thin film bilayers that interface a ferromagnetic insulator (FI), MgAlo.sFe1.504, with the canonical topological insulator (TI), BizSez. Although measurements of electrical transport (Hall effect) and polarized neutron reflectometry (PNR) provided the necessary indications of PME, we found that these were not sufficient. Nonlinear Hall effect data can be interpreted as signatures of an anomalous Hall effect or conduction of multiple carrier types due to interfacial charge transfer, while magnetic profiles deduced from PNR data can be modeled with either a MPE induced in Bi»Se, or a disordered layer at the interface. We used X-ray reflectivity and high resolution transmission electron microscopy (HRTEM) to show the presence of an interfacial layer between the Fl and TI, consistent with interdiffusion at the interface rather than a MPE in the BizSe3. Together, our results indicate that careful characterization of interfacial microstructure is essential in identifying a magnetic proximity effect in a topological insulator.