SUMMARY
I upgraded to KDE Plasma 6.5.3 and launched a new Plasma Wayland session via GDM (login manager). I was looking for a setting to disable the OSD notifications when I noticed that the CPU fan was getting louder, so I checked what was going on and found that something was eating my RAM. I managed to reboot the computer by pressing CTRL+ALT+DEL, launched a Plasma Wayland session and investigated what could have caused the memory usage spike. When I clicked on the notifications icon in the systray, I noticed that the CPU fan was getting louder again and I checked that the RAM consumption was increasing, so I concluded that the notifications widget caused the RAM usage spike. There weren't any notifications listed by the way.

STEPS TO REPRODUCE
1. Upgrade to Plasma 6.5.3 (I upgraded from 6.5.2)
2. Launch new Plasma Wayland session (I didn't test the X11 session)
3. Click on notifications icon in systray (the bell)

OBSERVED RESULT
Plasmashell starts to consume more and more RAM (in my case it reached 61,7 GiB according to the journal) and (I believe) the panel becomes unresponsive. CTRL+ALT+DEL still works if I press it immediately so I can reboot via the lockscreen (or whatever it is called).

EXPECTED RESULT
Plasmashell should not eat my RAM. ;-)

SOFTWARE/OS VERSIONS
OS: Arch Linux, Kernel 6.17.8-arch1-1, Mesa 25.3.0 on AMD Pitcairn GPU (radeonsi driver), amdgpu kernel driver (without DC enabled)
KDE Plasma Version:  6.5.3
KDE Frameworks Version: 6.20.0
Qt Version: 6.10.0