Is my iPhone 6s backlight filter broken?

Whilst replacing my iPhone 6s screen, the device was powered off. However I did not disconnect the battery. Whilst reconnecting the ribbon cables, the phone turned on accidentally and the screen flashed on for a split second before going dark. Everything else works, the phone is fully functioning including touch screen, but the display can now only be seen faintly when shining a light onto it leading me to believe the backlight filter has shorted/blown. The only issue is that after hard rebooting, the screen will flash on again fully for a split second. If the filter was broken, would it still do this?

I have checked vigorously for any loose connections or debris and fully disconnected/reconnected the new and old screens a number of times.

Thanks in advance!