SUMMARY:
I was trying to figure out why Spectacle is getting blurred screenshots and discovered an interesting problem, and now Spectacle is not guilty. Apparently, KWin has some rounding-related issues when it creates an image from the surface (window). It seems that when the window position can't be divided by the scale factor into an integer value, KWin returns a little bit blurred screenshot, which Spectacle saves as it comes. Maybe it also depends on the window size, but I'm not sure.

STEPS TO REPRODUCE:
1. Set the scale factor to 1.5.
2. Open some window, for example Konsole.
3. Put it at the 0x0 coordinates (top-left corner). Also, for just in case, ensure that the current window size can be divided by 1.5 without problems.
4. Take a screenshot and save it as a file.
5. Then move this window to somewhere to achieve an XY value that can't be divided by 1.5.
6. Take another screenshot and save it as a file.
7. Open it in GIMP and compare font glyphs.

OBSERVED RESULT
The screenshot is blurred

EXPECTED RESULT
Crystal clear screenshot whatever coordinates it was

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 6.0.2
KDE Frameworks Version: 6.0.0
Qt Version: 6.6.2
Kernel Version: 6.8.1-zen1-1-zen (64-bit)
Graphics Platform: Wayland
Processors: 16 × AMD Ryzen 9 7940HS w/ Radeon 780M Graphics
Memory: 28,2 GiB of RAM
Graphics Processor: AMD Radeon Graphics
Manufacturer: Micro Computer (HK) Tech Limited
Product Name: Venus series