If your display looks something like
Then that’s permanent pressure damage you’ve just applied to the monochrome screen. When you press on them hard the ink in the display gets all messed up. I’m unaware of a fix besides replacing the whole screen.
You can probably find a broken calculator on Ebay that has a display that’s fully functional (it’s pretty obvious if there’s pressure damage) then salvage the broken calculator’s display and replace your working calculator’s display with it. There’s a teardown guide here: TI-84 Plus Disassembly
Let me know if you have any other questions!