This occurs randomly and after 3-5 additional snapshots it may produce a non overlaid image. This also occurs with Gwenview and Dolphin of the items I can test. I have sent the full image as it shows only the dolphin part is affected. Delay does not seem to effect it.

STEPS TO REPRODUCE
1. start Spectacle
2. select "Take a New Snapshot"
3. the part of the Spectacle window overlays the snapshot area's image

OBSERVED RESULT
the part of the Spectacle window overlays the snapshot area

EXPECTED RESULT
image of window

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 5.25.5 with 5.19.0-23-generic (64-bit) Kernal
(available in About System)
KDE Plasma Version: 5.25.5
KDE Frameworks Version: 5.98.0
Qt Version: 5.15.6

ADDITIONAL INFORMATION
8 * AMD Rysen 7 4700, 15Gb, Renoir graphics processor, AZW PC, SER4 series