hello, do the buttons themselves show up? If they do, I wonder if it's because the letters were done with Inkscape's tools.
It could be because the glyphs are text, and the text depends on the system having the Poppins font. Try installing the font on your system, or install it to Godot if that's possible.
If you debug it or explore further, feel free to open an issue on the Github linked in the description!