EDIT: It seems the font/underscore/spaces issues should be fixed as of Geany verion 1.37: https://www.geany.org/documentation/faq/#geany-does-not-disp...
I'm not using Geany much at all any more, but still got love for it!
I have geany open right now and it is displaying underscores just fine with Noto Sans Mono. I have no idea what configuration you were using or what fonts, but this shouldn't be a problem currently.
I had the issue with the Flatpak version in an unmodified install on Fedora Silverblue 35, but my experience is apparently hardly unique.
[styling]
line_height=0;2;
I recommend Source Code Pro which is a great programming font and apparently doesn't have the problem, since I've never actually seen it.This has been a bug for years after they closed the issue and as far as I am aware they are the only software to have ever had a problem with these supposedly out of spec fonts that are still the default on some popular distributions.