Grey lines on dimmed display

Hi there! When woken up from stand by my Google Pixel 7a displays grey lines at approx. 90 % display brightness. The problem is less pronounced at full display brightness. However, at less than 100% brightness white spaces appear "washed out"/grey-ish, with horizontal lines respectively (see photo). The problem becomes more visible with an increasingly dimmed display. Intereslingly, at the colors and sharpness of photos seem fine at full brightness. What could be the cause of the problem? And how could it be fixed? Thanks for your help.