After updating mesa from 20.3.5 to 21.2.4 in bookworm my graphical KDE plasma 5.21 and now 5.23 under x11 renders unusable if in Settings > Display and Monitor > Compositor > Enable compositor on startup is turned off (and system restarted) . There are many more or less longer horizontal lines flying/flickering around, and changes of content, like typing text within a terminal, makes just written characters unreadable. The system is very slow to redraw the characters afterwards. Highlighting of elements like menus is slow and ugly (also with flickering lines). 

If 'Enable compositor on startup' is on, then there seem to be no problems. Also with mesa 20.3.5 and plasma 5.21 there are no problems, even with the swith off. Cannot check that for plasma 5.23, as it requires the new(er) mesa version.

In a similar but virtualized with kvm installation there seem to be no such problems, with all cominations of above mentioned mesa and plasma versions and switch on or off.

STEPS TO REPRODUCE
1. swtich off Settings > Display and Monitor > Compositor > Enable compositor on startup
2. restart
3. login
4. open a terminal and start writing or hover with mous cursors over menue items

Operating System: Debian GNU/Linux
KDE Plasma Version: 5.23.0
KDE Frameworks Version: 5.86.0
Qt Version: 5.15.2
Kernel Version: 5.14.0-2-amd64 (64-bit)
Graphics Platform: X11
Processors: 8 × Intel® Core™ i7-6700 CPU @ 3.40GHz
Memory: 31.2 GiB of RAM
Graphics Processor: Mesa Intel® HD Graphics 530 (rev 06)

$dpkg -l | grep mesa
ii  libegl-mesa0:amd64                            21.2.4-1                            amd64        free implementation of the EGL API -- Mesa vendor library
ii  libgl1-mesa-dri:amd64                         21.2.4-1                            amd64        free implementation of the OpenGL API -- DRI modules
ii  libgl1-mesa-glx:amd64                         21.2.4-1                            amd64        transitional dummy package
ii  libglapi-mesa:amd64                           21.2.4-1                            amd64        free implementation of the GL API -- shared library
ii  libglu1-mesa:amd64                            9.0.1-1                             amd64        Mesa OpenGL utility library (GLU)
ii  libglx-mesa0:amd64                            21.2.4-1                            amd64        free implementation of the OpenGL API -- GLX vendor library
ii  libosmesa6:amd64                              21.2.4-1                            amd64        Mesa Off-screen rendering extension
ii  mesa-utils                                    8.4.0-1+b2                          amd64        Miscellaneous Mesa GL utilities
ii  mesa-va-drivers:amd64                         21.2.4-1                            amd64        Mesa VA-API video acceleration drivers
ii  mesa-vdpau-drivers:amd64                      21.2.4-1                            amd64        Mesa VDPAU video acceleration drivers
ii  mesa-vulkan-drivers:amd64                     21.2.4-1                            amd64        Mesa Vulkan graphics drivers

My original debian bug entry can be found here: https://bugs.debian.org/998386

Thanks,
Chris