If you are confident that the battery was disconnected before removing the screen and only reconnected after the screen, then I would double check the condition of the connectors, the flexes and the small components surrounding the connectors. You may want to use magnification unless you have young eyes :>).
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If you have second thoughts (about the battery), this does look like a backlight circuit issue and this would be repairable via a micro-soldering repair.
Beyond that, I would try to re-install the original screen if it is still in working condition. You may have a bad re[placement screen.
If you are confident that the battery was disconnected before removing the screen and only reconnected after the screen, then I would double check the condition of the connectors, the flexes and the small components surrounding the connectors. You may want to use magnification unless you have young eyes :>).
Beyond that, I would try to re-install the original screen if it is still in working condition. You may have a bad re[placement screen.