That is not affecting the backlight. The crack is an screen protector.
The issue is that the colros, contrast and brightness of the screen gets affected and you can see horizontal lines. The screen is in good physicall conditions. It must be drivers issues. It started just after the last security update the phone received.
Maybe its true..my phone also got this horizontal line and the screen become abit yellowish colour. At first i taught the crack is on screen protector but then i remove the screen protector and saw a crack at the top right edge is broken, and the crack line continue along the screen.
So maybe this is the reason? If this true, anyone know how to fix this problem? Or this horizonyal line happens because of something else?
have you tried rebooting the phone to safemode?
HOLD the POWER BUTTON> tap and hold the Shudown icon till safemode appears>> tap safemode
If you are successful a word " safemode" appear on the lower part of the screen, then try to check if you have the same issue
If the phone on safemode, it will disable any 3rd party apps installed on the phone and will use only the pre isntalled apps, including the theme
now if the horizontal line dont appear, try to uninstall thems,wallpaper or app that may cause the horizontal line
If all fails, backup the phone and check if theres any service center near your area
but most of the service location is closed due to COVID-19 issue, you might need to mail in the device.
It did not worked. on safe mode or after factory reset. The phone recovered the screen by it selft hours after resetting it to factory. The issue now looks to be intermitent.
I would try taking the device to a UBreakIFix repair center for a technician to take a better look at the symptoms you are experiencing.
Make sure the device is backed up so you won't lose any important data during the evaluation.