SUMMARY
https://bugs.kde.org/show_bug.cgi?id=423594 looks to be back.
I just encountered it about 1 hour after upgrading to 5.21.

STEPS TO REPRODUCE
1. Get Chrome "Canvas Blocker" extension to pop up a few or more notifications.

OBSERVED RESULT
The subtle countdown/close indicator in the notification freezes and I hear my cpu fan roaring (although I'm compiling kodi and the kernel at the same time).
htop shows plasmashell peaking the cpu.
I also couldn't "--replace" plasmashell. I had to kill it.

EXPECTED RESULT
Just show the notification and don't hang (but still be cool).

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Arch Linux
(available in About System)
KDE Plasma Version: 5.21.0
KDE Frameworks Version: 5.79.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
It is by no means a frequent issue, and I get a ton of notifications (mostly the canvas blocker, and only 3 this time).
This could very well be the only other time I've encountered it since I posted about it in the previous report.

Nonetheless, the backtrace:
#0  0x00007661390589ba in __futex_abstimed_wait_common64 () at /usr/lib/libpthread.so.0
#1  0x0000766139052260 in pthread_cond_wait@@GLIBC_2.3.2 () at /usr/lib/libpthread.so.0
#2  0x000076613a01c0c4 in QWaitCondition::wait(QMutex*, QDeadlineTimer) () at /usr/lib/libQt5Core.so.5
#3  0x000076613bc5c137 in  () at /usr/lib/libQt5Quick.so.5
#4  0x000076613bcc5b87 in QQuickWindow::event(QEvent*) () at /usr/lib/libQt5Quick.so.5
#5  0x000076613ae9c752 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /usr/lib/libQt5Widgets.so.5
#6  0x000076613a1fda2a in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /usr/lib/libQt5Core.so.5
#7  0x000076613a5c6319 in QPlatformWindow::windowEvent(QEvent*) () at /usr/lib/libQt5Gui.so.5
#8  0x000076613aea3437 in QApplication::notify(QObject*, QEvent*) () at /usr/lib/libQt5Widgets.so.5
#9  0x000076613a1fda2a in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /usr/lib/libQt5Core.so.5
#10 0x000076613a255a15 in QTimerInfoList::activateTimers() () at /usr/lib/libQt5Core.so.5
#11 0x000076613a2562fa in  () at /usr/lib/libQt5Core.so.5
#12 0x00007661385dfb84 in g_main_context_dispatch () at /usr/lib/libglib-2.0.so.0
#13 0x0000766138633c21 in  () at /usr/lib/libglib-2.0.so.0
#14 0x00007661385de3b1 in g_main_context_iteration () at /usr/lib/libglib-2.0.so.0
#15 0x000076613a256691 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
    at /usr/lib/libQt5Core.so.5
#16 0x000076613a1fc3ac in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () at /usr/lib/libQt5Core.so.5
#17 0x000076613a204844 in QCoreApplication::exec() () at /usr/lib/libQt5Core.so.5
#18 0x00005e7727121069 in  ()
#19 0x0000766139bc3b25 in __libc_start_main () at /usr/lib/libc.so.6
#20 0x00005e77271212ae in  ()