Replaced screen after 10-15 secs screen brightness lowers

Hi all,

So spent some time hunting and cant seem to find a definitive answer.

So just replaced an iPhone 8 screen, phone boots normally and with normal brightness, then after approx 10 secs the brightness falls and can just about make out the screen contents. Brightness settings are still on full when this happens so cannot increase brightness.

Re-installed the original screen and no issue at all works perfectly. I thought maybe a bug but that seems to have been fixed with the iOS updates so did a hard reset, still the problem persists.

Any help will be appreciated

Thanks