I'm willing to bet this is related to the ambient light sensor. It's an issue I have seen myself with the iPhone 12. If the Ambient Light senor isn't working properly this is exactly the behavior you'll get. Apple logo is maximum brightness, because it's always maximum brightness. But once the lock screen shows, the ambient light sensor kicks in. If it's not working properly, then the screen will always be at minimum brightness.
Did you transfer the sensor/receiver speaker cable from the old screen? If so those cables are very delicate and it's possible it was damaged. It could even have been damaged beforehand if the old screen was damaged in that area. I could give it a good inspection. The iFixit Guide even calls this out in this step of the screen replacement guide.
If the new screen came with a new sensor cable, it could just be that the ambient light sensor is a dud.
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