Dark spot on iPad Pro screen

I have exactly the same problem, also iPad Pro 12.9” first saw it yesterday about 2:40. I installed a plain white wallpaper to check more clearly and I cleaned the screen thoroughly and was most upset when the spot was still there. I did a backup, installed the latest IOS update and made an appointment at the Genius Bar for Tuesday. Also the same, a screenshot didn’t show it so I too took a photo with my phone. The spot was still there at this stage, however when I picked it up again later last night the spot was gone, I kept checking and thought I should wait till daylight to check it out, mysteriously it is still gone. I thought it had something to do with the assistive touch shortcut, but was astounded to see someone else with the same problem. I don’t know whether to cancel my appointment or not. Also I have just tested and can also replicate the spot by pressing on the back.