I'm taking about the special characters you can insert when pressing for examplealt+6+4 and in return you get the@ character. I don´t know if it is a bug or a missing feature.
This is a link of what I'm refering to:

https://en.wikipedia.org/wiki/Alt_code

Some times I use alt codes to insert special character and also to insert characters that are missing between keyboard layouts like ANSI and ISO.

If I open stock neovim in the terminal, the alt commands work perfectly.