I found a solution and it worked for me!
I recently bought a 4th Gen iPad Pro 12.9 inch with a "defective screen" according to the seller. Apparently he got a new iPad Pro, so the unit is just lying around. I got it at a really cheap price. When I looked at it, the screen has 2 blueish/ dark violet lines across the screen. I thought at first they are dead pixels, but the colors are not totally black, which means the pixels are not totally dead. They are still emitting light. if it is not black, it is only a stuck pixel. The seller also said that the lines would sometimes disappear (like 5% of the time). It disappears for a few seconds and then would reappear back once the screen shows white colors.
So I tried scouring the internet looking for a possible solution and I found one that worked for me. So, there are "stuck pixel fixers" on youtube and all I did was play it for 3 hours, and viola! The line was GONE!!! Just play the video in full screen and zoom in the video (via pinch-zoom), so that the entire video is covering the entire screen (no black bars at the top). Leave it for a couple of hours. Don't stare at it because it's painful in the eyes. Also, turn the brightness of the iPad up. Mine was set at 80% brightness when I did it. Hope this also works for you.
I was so pumped. I only spent $300 for an iPad Pro 12.9 with 256 GB Storage and Cellular capability, which could easily sell for $500-$700. Not selling it though. ^_^
Here is a link to the video I used to fix it.
Cheers!