SUMMARY
XWayland windows are either completely invisible or empty in Plasma 6 Wayland.

STEPS TO REPRODUCE
1. Log into a Plasma Wayland session
2. Launch any app that doesn't have native Wayland support (I ran glxgears and xclock from the terminal to reproduce this)
3. Notice that either an empty window or nothing appears on the screen

OBSERVED RESULT
Either an empty window with decorations and a completely transparent background or nothing appears. However, the app icon is still shown properly in task manager widgets.

EXPECTED RESULT
The window appears normally with its contents and decorations

SOFTWARE/OS VERSIONS
Operating System: KDE neon Unstable Edition
KDE Plasma Version: 5.90.0
KDE Frameworks Version: 5.246.0
Qt Version: 6.6.0
Kernel Version: 6.2.0-37-generic (64-bit)
Graphics Platform: Wayland
Graphics Processor: SVGA3D; build: RELEASE; LLVM;
Manufacturer: innotek GmbH
Product Name: VirtualBox
VirtualBox Version: 7.0.12
Host OS: Manjaro Linux

ADDITIONAL INFORMATION
When you click on the XWayland app icon in the task manager, it won't focus the app.

This also affects the Calamares installer on the live ISO for Neon Unstable Edition. Quite strangely, I was still able to interact with the exit confirmation dialog when I clicked Close on the main installer window. I had to disable automatic login and switch to an X11 session in the live session to be able to install the system on my VM.