SUMMARY
Screenshot preview shows right screen again in right corner of right screen.
It's hard to describe.
Imagine the screen looks like that before opening spectacle:

|------------------|
|  x                  |
|                      |
|------------------|

After opening spectacle the preview looks like this:
|------------------|
|  x       -------- |
|          |  x      |
|------------------|

So the contents of the upper from the upper left corner are shown again overlaying the rest of the screen.

Here is are real screenshots:
https://imgur.com/a/QIckubF

  
STEPS TO REPRODUCE
1. Configure Spectacle to allow for free selection on hotkey press 
2. press hotkey to take a screenshot

OBSERVED RESULT
The preview will be broken, as described above.
The screenshot will be of the original screen, the new overlayed image isn't captured. So it is possible to capture these areas, but you need to do it blindly.

EXPECTED RESULT
A normal preview of the screen to select the target region.


SOFTWARE/OS VERSIONS

Linux/KDE Plasma: NixOS with KDE 
KDE Plasma Version: 5.27.10
KDE Frameworks Version: 5.112.0
Qt Version: 5.15.11

ADDITIONAL INFORMATION
Kernel Version: 6.1.66 (64 bit)
Graphics Platform: X11
GPU: Nvidia RTX
CPU: Amd Ryzen