SUMMARY
So i have constant high Memory and Swap usage. My swap is zram-only, no hard-drive backed swap. Usually swap doesn't reach 100%, but today it reached. So the RAM started going up after that and once it reached 90% early-oom kicked in and killed Firefox.
After Firefox was killed the Konsole window (and i guess other apps that were opened at that moment) became transparent. I don't remember about the other apps because this happened yesterday and i'm filling the bug today based only on what i see recorded. I'm pretty sure other apps opened were affected too, because it's not the first time this happened and the other times it happened it affected all opened apps at the time but not newly opened ones after that.
It's not only from KDE 23 or so, it's from before. I don't know what makes this happen and early-oom killing/high swap might be or might not be the reason.
I checked the early-oom log and the only thing it killed was GeckoMain from Firefox, nothing else, let alone kwin or plasma-related stuff. Also not sure if the other times it happened early-oom was killing something.
I don't know how to reproduce but nevermind i file this bug. I'll try to check if/when it happens again whether earlyoom killed something at that time.

Frequency of appearance of this bug i believe is something like 1 time a month +- a week, based only on my foolish memory so take it with a grain of salt.
OBSERVED RESULT
Please see the screenshot here: https://youtu.be/gza8QU5fNko

EXPECTED RESULT
No transparent windows glitch

SOFTWARE/OS VERSIONS
Operating System: Kubuntu 21.10
KDE Plasma Version: 5.23.1
KDE Frameworks Version: 5.87.0
Qt Version: 5.15.2
Kernel Version: 5.13.0-20-lowlatency (64-bit)
Graphics Platform: Wayland
Processors: 8 × Intel® Core™ i5-8250U CPU @ 1.60GHz
Memory: 7,6 GiB of RAM
Graphics Processor: Mesa Intel® UHD Graphics 620