Some applications missing icon on Wayland

Under Wayland, some applications end up having a missing or broken icon. I have observed this on both Budgie Wayland using XFW, and the XFCE 4.20 pre-releases. Icons are retrieved correctly in both cases under X11. It happens both when using xfw_application_get_gicon() and xfw_application_get_icon(). I am running this all on Solus, and Joshua Stobl is also experiencing this on Fedora Rawhide.

Observed affected applications:

Zen browser (flatpak)
CLion (Jetbrains Toolbox)

I first thought that maybe it has to do with flatpak, or third-party distribution channels in general, but the Spotify flatpak icon works just fine, so it's not that simple.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information