I can confirm your Problem.
My system is
Operating System: openSUSE Tumbleweed 20230520
KDE Plasma Version: 5.27.5
KDE Frameworks Version: 5.106.0
Qt Version: 5.15.9
Kernel Version: 6.3.2-1-default (64-bit)
Graphics Platform: X11
Graphics Processor: Mesa Intel® HD Graphics 630
# zypper se -si spectacle
Loading repository data...
Reading installed packages...
S | Name | Type | Version | Arch | Repository
---+----------------+---------+-------------+--------+------------------------
i+ | spectacle | package | 23.04.1-1.1 | x86_64 | openSUSE Tumbleweed Oss
i+ | spectacle-lang | package | 23.04.1-1.1 | noarch | openSUSE Tumbleweed Oss
#
So far I have found no solution to the problem but a workaround:
I disable the compositor:
systemsettings5 → Display and Monitor → Compositor disable Compositing: Enable on startup
This gives me back the old behavior of the spectacle selection area (bright on dimmed background).