Edit: Found the issue! There is a small rubber piece that sits in the hole where the ambient light sensor is located, on my original (broken) display frame. This rubber piece was not included on my replacement display assembly frame, and no guides mentioned moving this piece into a new frame. I opened my phone back up and moved this rubber piece and it resolved the issue.
I am having the same issue. I just replaced my LCD assembly today. When the ambient light sensor detects light, everything is fine, the second screen stays on and I'm not experiencing any issues. In a dark room however, the second screen turns off, and the following issues happen as well. The phone seems to go into like a "deep sleep" and waking the phone is difficult. Knock On does not work. Fingerprint sensor to unlock does not work. Pressing the power button only works sometimes, after multiple presses. Even when my screen was broken, I had none of these issues. Since replacing the LCD though, these issues started. I replaced the screen myself.
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