Created attachment 142113 [details]
Screen recording of the animation for the Desktop Grid effect.

SUMMARY
When the Desktop Grid effect is triggered from a keyboard shortcut or a screen edge, a part of the wallpaper on each desktop is replaced by a black region during the animation, which is disorienting. Everything is fine once the animation ends.

STEPS TO REPRODUCE
1. Have more than one virtual desktop
2. Trigger desktop grid effect using a keyboard shortcut

OBSERVED RESULT
During the animation, part of each desktop's wallpaper is replaced by a black region. I have attached a screen recording (after setting the animation speed (System Settings > Workspace Behaviour > General Behaviour) to the slowest possible value) which displays this behaviour. The desktops are displayed correctly when the animation ends.

EXPECTED RESULT
No graphical glitches during animation.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux
KDE Plasma Version: 5.22.90
KDE Frameworks Version: 5.86.0
Qt Version: 5.15.2
Kernel Version: 5.10.70-1-lts (64-bit)
Graphics Platform: X11
Processors: 4 × AMD EPYC-Rome Processor
Memory: 1.4 GiB of RAM
Graphics Processor: virgl (this is a VM, but the bug is also reproducible on bare metal with AMD graphics)

ADDITIONAL INFORMATION
The same glitch happens in reverse when exiting the Desktop Grid effect