SUMMARY
If I choose to mirror the laptop display to an external one, the 'present windows' desktop effect shows no windows. However, I can still switch windows using Alt+Tab.

STEPS TO REPRODUCE
1. Connect an external monitor
2. Choose to mirror the existing display on the OSD that appears with various display options
3. Trigger the 'present windows on current desktop' effect (I do this through a screen edge)

OBSERVED RESULT
No windows are shown, just an empty desktop

EXPECTED RESULT
Windows should be shown in the 'present windows' effect

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 6.1.4
KDE Frameworks Version: 6.5.0
Qt Version: 6.7.2
Kernel Version: 6.6.49-1-lts (64-bit)
Graphics Platform: X11
Processors: 8 × AMD Ryzen 5 3500U with Radeon Vega Mobile Gfx
Memory: 21.3 GiB of RAM
Graphics Processor: AMD Radeon Vega 8 Graphics
Manufacturer: LENOVO
Product Name: 20NES02N00
System Version: ThinkPad E495

ADDITIONAL INFORMATION
A. The windows are still shown outside the effect, and I can switch windows through Alt+Tab.
B. The effect works correctly without an external monitor.
C. The 'Desktop grid' effect also shows no windows when this bug is triggered
D. Kwin support information (without the external monitor connected): https://invent.kde.org/-/snippets/3227