SUMMARY
I've got a 4K monitor and a 1080p one and I use xrandr to scale the 1080p display. But then after I click 'Take a snapshot', all displays just get messed up with contents showing in wrong monitors. It's again normal after quitting the snapshot mode though.

STEPS TO REPRODUCE
First of all, my display layout:
Display layout:
┌----------------┐┌--------------------------┐┌------------------┐
| 1920x1080 DP-0 ||                          || 1920x1080 HDMI-0 |
└----------------┘|      3840x2160 DP-3      |└------------------┘
                  |                          |
                  └--------------------------┘
1. Run 'xrandr --output DP-0 --auto --left-of DP-3 --output DP-3 --auto --primary --output HDMI-0 --auto --right-of DP-3 --scale 2.0' (I put it in SDDM Xsetup)
2. Launch Spectacle
3. Click 'Take a snapshot'

OBSERVED RESULT
Display layout becomes sort of messy and broken in snapshot mode.

EXPECTED RESULT
All contents in all monitors remain untouched in snapshot mode.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 5.17.1
KDE Frameworks Version: 5.63.0
Qt Version: 5.13.1
Kernel Version: 5.3.7-arch1-1-ARCH
OS Type: 64-bit

ADDITIONAL INFORMATION
GPU: Nvidia GTX980M
Driver: Nvidia proprietary
Compositor scale method: Accurate
Compositor backend: OpenGL 3.1