There must be more to this. I just launched a terminal and created a file to test with nvim on arch and it works perfectly fine.

Take a file, sudo chown root:root filename, sudo chmod 700 filename, edit with nvim and save with :w !sudo tee % then reload. Works fine.

I’m on arch with suckless st.

Edit:

Made a demo vid - https://youtu.be/YKZuAvoSW5g