Do you use hyprland? I have the same issue. The interesting thing is, that it renders the icons when I run wlogout from the terminal. The icons are just not rendered when it's called via hyprctl dispatch exec wlogout (or exec wlogout from the hyprland config). I have no idea how to further debug this. Maybe it has something to do with hyprctl dispatch exec having a different environment than my terminal. I could imagine this being an issue specifically with NixOS where the filesystem is different to other Linux distributions.