Flathub recently changed its icon, but Discover is still using the old one (see attached) when interacting with apps from Flathub.
I think Discover should be update to match the new one on Flathub website.
I think you should notify the KDE Developers at https://bugs.kde.org.
Discover shows the icon provided by Flathub itself:
https://dl.flathub.org/repo/logo.svg
You can verify it from their flatpakrepo file (open in Kate, or another text editor)
https://dl.flathub.org/repo/flathub.flatpakrepo
Discover uses whatever icon a flatpak repo provides, for example, if you add elementary OS’ appcenter, they will show their logo instead
Maybe you can try notifying Flathub’s maintainers about this