My ipad has 6 has dim spots on the side and issue with brightness

Ipad gen 6 (ipad 6) is having issues with display with dim spots in the sides which can be visible in a white background and also while using the brightness fluctuates when using in full brightness. The device has no water damage or any drops in its entire life . The spots have been present in my 3 of my friends ipad now 4 ipads with same issue and these spots can be easily recognised in white background during night time and when we lower the brightness