Here's a screenshot, check out the top right corner:

https://i.imgur.com/pgx46We.png

How to reproduce:

1. save this package which makes org tables pretty by putting unicode characters onto regular org table characters

https://raw.githubusercontent.com/Fuco1/org-pretty-table/master/org-pretty-table.el

2. eval it, so you get org-pretty-table-mode
3. download the attached org file
4. open it and go to the first character of table
5. turn on org-pretty-table-mode, the table should be switched to pretty mode
6. before the first character of the table type a space, so the header row is shifted by one character to the right
7. the display artifacts should appear at the top of the window
8. if you press cursor up until the window starts scrolling then you can see parts of the artifact staying on the screen during scrolling

Pretty table mode uses jit for highlight, it might be an issue with jit. I assume it's some kind of emacs problem, because of the artifact staying in place while scrolling.

I tested it on Windows: GNU Emacs 27.1 (build 1, x86_64-w64-mingw32) of 2020-08-12