Bug 495474

SUMMARY
As you can see in the screenshot, a lot of applications have generic Wayland icons in their thumbnails when switching windows with Alt+Tab.

All of them except for OpenSCAD, have their own displayed as expected in the task manager at the bottom of the screen. So, that means those applications do define a custom icon in a way that is recognized by the system. Even assuming there's a separate way for specifying an icon in a different context (i.e. thumbnail in window switching), that shouldn't be required. If you don't have a specific icon for that you should use the main icon by default.

STEPS TO REPRODUCE
1. have several apps open, including Ultimaker Cura, Mozilla Thunderbird, Pamac (!!!)
2. hit Alt+Tab

OBSERVED RESULT
some windows have generic wayland icons

EXPECTED RESULT
all windows should have their icon as defined by the application, which is correctly displayed in the task manager

SOFTWARE/OS VERSIONS
Operating System: Manjaro Linux 
KDE Plasma Version: 6.1.5
KDE Frameworks Version: 6.6.0
Qt Version: 6.7.2
Kernel Version: 6.6.54-2-MANJARO (64-bit)
Graphics Platform: Wayland
Processors: 12 × 12th Gen Intel® Core™ i7-1255U
Memory: 15.3 GiB of RAM
Graphics Processor: Mesa Intel® Graphics
Manufacturer: ASUSTeK COMPUTER INC.
Product Name: Vivobook_ASUSLaptop X1502ZA_F1502ZA
System Version: 1.0