If it was not don'e by an apple certified repair shop then there is almost a certainty that it is an aftermarket screen,. These screens rarely have the same color temperature and resolution as the original ones. You can try to adjust the temperature a bit using nightshift mode or by adjusting the white balance and transparency in the accessibility options in settings
There's a few things that can cause this and a few solutions. First, you say one corner is bumped a little. If this is the bottom corner to the right of the home button then it means the digitizer flex cable has not been folded under properly and need to be adjusted. Glue not sticking could be a few problems. The most likely is the frame not being clean enough, the other is poor quality adhesive. It also could be that it has been handled too much. In my shop my process is to warm the adhesive a bit with my heat gun before I press the digitizer into place, then once it is in and sitting flush I use the black paper clamps to keep it held tight for 1-2 hours so the adhesive can set well. Warning, DO NOT place a clamp over the home button or within an inch of it on each side as the glass there is quite easy to break. If your adhesive is now not sticky due to too many attempts, I'd suggest removing it and using 3M double sided adhesive (9448A) Hope this helps
The remedy is a weird one. You must remove the logic board. When you do this you will see the proximity sensor on it has a black rubber covering with two holes. One big and one small. Take the rubber covering off and flip it. I'm not sure why it works, but I've used this to fix this problem dozens of times
Most times when I come across this it is a buildup of dirt and oils in the mesh covering the ear speaker on the front panel. Start by removing the earspeaker. Once you have it exposed use a q-tip with isopropyl alcohol to moisten the buildup in the mesh. Then use a can of compressed air and the sharp tip of the spudging tool to scrape and blow away any buildup. You can hold the panel up to the light to see your progress. It may take a few soakings and shots of compressed air to free all of the buildup.
My guess is damage to the LCD. This doesn't show up on the screenshot because the logic board doesn't know that the screen is damaged. Replacing the screen will likely solve it, or at least tell you if the problem is the LCD or the logic board connections.
First suggestion. Plug in the iPad with the lightning cable connected to the wall adapter. Then hold the home and sleep/wake buttons simultaneously for 15-17 seconds until the phone reboots. This solves a surprising amount of glitches.
This is most likely a damaged LCD. The way to tell if it's the backlight is shine a flashlight on the screen when you think it's on and see if you can see faint images. If you don't see anything, it's the LCD