Invisible loading indicator (GtkSpinner) for GTK apps

I have developed my own custom gtk3 app in python, and I’m running into this issue. Here is gtk3-demo for example:

I’ve configured my gtk theme with home-manager like so:

  gtk = {
    enable = true;
    iconTheme = {
      package = pkgs.gnome.adwaita-icon-theme;
      name = "Adwaita";

If its not the icon theme that is causing this issue, then what could it be?