I noticed that there are many people experiencing the same issue:
Getting more frequent “A stop job is running” notices than before - Support - Manjaro Linux Forum
kwin_x11 and QDBusConnection hanging on shutdown/reboot / Newbie Corner / Arch Linux Forums
So far I only found a workaround to KILL kwin immediately during shutdown/reboot process.
Kwin_x11 not stopping during shutdown - Support - Manjaro Linux Forum
However, it doesn’t explain why kwin is stuck. Is it a bug of kwin? Wondering if anyone reported it yet. 