A 68-year-old man from Delhi presents with progressive vision loss in both eyes over 2 years. He has a history of poorly controlled type 2 diabetes (HbA1c 9.2%) and hypertension. On examination, visual acuity is 6/60 in both eyes. Slit-lamp examination reveals a dense nuclear sclerotic cataract in both eyes. He undergoes phacoemulsification with IOL implantation in the right eye. Three weeks post-operatively, his refraction shows a hyperopic shift of +1.5 D compared to the pre-operative target. The IOL power used was 20 D. Which of the following is the most likely cause of this refractive error?