Probably a bug in gnome-screenshot. After a bit of a search - there were plenty of bugs that mentioned graphical glitches after taking a screenshot.
Glad updating to Debian 11 fixed it!
Personally, I use scrot (fast and lightweight, terminal based screenshot software) in a short script I wrote.
And then in my WM (dwm - a tiling WM) - I’ve bound a keybind to run the script and take a screenshot. I’ve used scrot for years.
I find gnome screenshot extremely annoying. Especially its pointless flash and shutter noise. Completely unnecessary!