Rather sounds like part of the backlight isn't on. It might help if you could add some pictures of what you're seeing to your question.
Adding images to an existing question - iFixit Repair Guide
There are a couple of possibilities here. The most likely and easiest is to double check the two connectors going to the screen; one is the digitizer or touch screen and the other is for the LCD. If the LCD connector isn't seated correctly, it can cause all kinds of various problems like the one you're seeing. Unplug them both (I'm not sure which is which on your phone) and check them carefully with a magnifying glass and a bright light. Look for any bent or broken pins on either side; sometimes bent ones can be straightened with fine tip tweezers. Check along the flex cables themselves for any signs of damage like being torn or sharply creased.
Once you're satisfied everything's okay, clean both sides with 90% or higher concentration isopropyl (rubbing) alcohol and let it dry. Plug everything back in and recheck to see if it's working. Pay special attention to the note on the step where you disconnect the display cables.
To re-attach press connectors like this one, press down on one side until it clicks into place, then repeat on the other side. Do not press down on the middle. If the connector is even slightly misaligned, the connector can bend, causing permanent damage.
Like I said, that's the most likely scenario; otherwise we have seen a lot of issues where backlight circuits on either the display or the logic board have been damaged by plugging and unplugging the display with the battery still connected. You always want to unplug the battery first before you do anything else inside the phone to avoid exactly that kind of problem. Usually replacing the display fixes it, but sometimes it requires board level repairs which can get expensive.
Those are the most likely scenarios; hopefully it just a case of a misaligned connector and reconnecting it will make the problem go away. Like I said, pictures would help, but check out the connectors and see if that helps.
Good luck; let us know what you find!
Okay, the picture makes it much clearer. From what I've seen of all kinds of broken screens, this isn't a connector or a backlight issue. I really don't know how that might have happened, but my best guess is that somehow the LCD has become delaminated. I wonder if perhaps it got flexed a little too much in the process of removing it from the frame, is all I can think of.
I suggest that your next step is to go ahead and replace the screen. Since you've removed it once already I assume you don't need me to point out the guide. LCD screens are getting dirt cheap these days with everyone going to OLED. One of the first ones I saw on Amazon was only $13 for a brand new screen, so I'd say you should get one like that to fix the problem.
Amazon.com: Screen for iPhone 8 Plus Replacement 5.5 inch 3D Touch LCD Digitizer Display Frame Assembly with Free Repair Tool Kit for A1864, A1897, A1898 (White) : Cell Phones & Accessories