They aren't his glasses
In B/W, Cheren constantly wears reading glasses because of how often he studies pokemon strategies and the like. By the time B2/W2 roll around he already has enough knowledge and experience to be a gym leader so he no longer needs to read up as often. Hence, no glasses.
Bianca gains glasses because she has poor eyesight which was overlooked due to her sheltered lifestyle. In B/W, Bianca is quite clumsy at times, a trait of having blurred vision. In B2/W2 she has a pair of corrective lenses and is no much more stable on her feet.
Cheren x Bianca is cute but it just isn't meant to be, OP
