Epitaxial thin films of the La_1–xSr_xMnO₃ system, spanning a wide range of compositions (0.5 ≤ x ≤ 0.65), have been prepared, using the polymer-assisted deposition method, on SrTiO₃ (100) substrates. The primary objective was to achieve the highly stable A-type antiferromagnetic (AF) phase associated with x = 0.5. However, the electronic and magnetic properties of the samples, with different substitution levels of La with Sr, exhibit deviations from the anticipated behavior according to the bulk phase diagram. Employing X-ray absorption spectroscopy, we demonstrate that the effective 0.5:0.5 ratio of Mn³⁺:Mn⁴⁺ is actually attained in the sample with x = 0.65, suggesting the presence of an alternative charge compensation mechanism. High oxygen pressure annealing processes allow us to demonstrate that oxygen vacancies, generated to accommodate the epitaxial structural strain, are responsible for the partial charge compensation and the observed deviations from the bulk phase diagram.