Haven’t seen it indeed. Afaik, app icons should be fine. In case they are not, I have an idea, what the issue might be. Thank you.
Reading through the bug tracker, the GNOME approach seems to be one I see way to often: “The standard is outdated in our view and does not suit us. Let’s just treat it as obsolete and make our own incompatible thing. If this breaks stuff, its not our problem.” I prefer those that say: “We need an updated standard.” But they seem to be the minority, at least these days.
That is my impression as well.
Just for completeness: According to GNOME Human Interface Guidelines app icons are full-color icons: https://developer.gnome.org/hig/guidelines/ui-icons.html My app should be fine! 😊