Created attachment 186537 [details]
screenshot

SUMMARY
When building the application as Flatpak, using setPixmap in notifications causes the application icon in the notification header to disappear. Instead, a plasma-workspace icon is displayed.
This happens because in the Flatpak environment, the pixmap handling mechanism may conflict with the display of the application icon.
I create a testing application: https://invent.kde.org/saurov/test-flatpak-notifications

STEPS TO REPRODUCE (on my application)
1. Build the application as Flatpak:
flatpak-builder build --user --install-deps-from=flathub --force-clean --ccache --install .flatpak-manifest.json

2. Run the application:
flatpak run org.kde.test-flatpak-notifications

3. Click the "Notification with pixmap" button

OBSERVED RESULT
The application icon is missing from the notification header.

EXPECTED RESULT
The application icon should be displayed in the notification header even when using pixmap.

SOFTWARE/OS VERSIONS
Operating System: CachyOS Linux 
KDE Plasma Version: 6.5.1
KDE Frameworks Version: 6.19.0
Qt Version: 6.10.0
Kernel Version: 6.17.7-3-cachyos (64-bit)
Graphics Platform: Wayland
Processors: 12 × Intel® Core™ i5-10500H CPU @ 2.50GHz
Memory: 16 ГиБ of RAM (15.4 ГиБ usable)
Graphics Processor 1: Intel® UHD Graphics
Graphics Processor 2: NVIDIA GeForce RTX 3060 Laptop GPU

ADDITIONAL INFORMATION
This trouble also present in other KDE apps if use Knotifications (flatpak version) e.g. NeoChat.
And all flatpak apps that use Knotifications do not have KCM app notification settings.