Impossible to set shortkey on Shift+Ctrl+Alt+8

Does anyone know why I can’t bind any key to Shift+Ctrl+Alt+8 in the Writer keyboard window? It lets me bind Shift+Ctrl+Alt+6/7/9, etc, but not 8. Same for Shift+Ctrl+8. But Ctrl+8 or Alt+Ctrl+8 works fine!

It seems like the key isn’t getting to Openoffice at all, because the key window lets you press a hotkey to jump to that key selection. They all jump to the right place except this one.

I’m on linux (Mint 20.3).