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Lattice defects such as oxygen vacancies, interstitials, and their complexes are present in crystalline oxide materials. In particular, anti‐Frenkel defects, which refer to charge‐neutral anion vacancy‐interstitial pairs, are strongly coupled with ferroelectric and dielectric properties as electric dipoles. However, in order to observe their macroscopic manifestation, delicate defect controls are required to the extent that electronic and ionic charges are almost completely suppressed. Here, the thermal cycle dependence of dielectric and piezoelectric properties is scrutinized in the strain‐driven morphotropic phase boundaries of multiferroic La‐substituted BiFeO3 thin films. Electrochemical impedance spectroscopy provides the Warburg feature that is considered evidence of the ionic origin. The observations are discussed based on anti‐Frenkel defects that are created or annihilated reversibly by thermal cycles through high‐temperature structural phase transition temperature or magnetic Néel temperature. The defect dipoles are spontaneously aligned by the flexoelectric effect in the phase boundaries inducing a metastable interfacial ferroelectric phase. The findings offer useful insight into defect dipoles. [ABSTRACT FROM AUTHOR]