All right guys. I solved this problem. Please refer to this: applications-icon-missing-in-system-tray with detailed solution included.
1. Rename ~/.config to something else like ~/.config.backup
2. Reboot
3. Check if Cookies opens normally (or any of your other issues don’t exist).
4. If everything works, move a few files from ~/.config.backup back to ~/.config.
5. If something break, now you know which file/dir is the cause.
6. Repeat from 2 till ~/.config.backup is empty.
7. Profit!
Finally I found ~/.config/kwinrc is the culprit, which caused another issue I encountered before: AX210 cannot connect wifi normally. Yes this issue was one of symptoms.
Actually I found many "apps not showing in the system tray"-like issues in forums or manjaro forums. Maybe kwin blocked the communication between DBus and other apps, including wifi...?
My ~/.config/kwinrc is below:
[$Version]
update_info=kwin.upd:replace-scalein-with-scale,kwin.upd:port-minimizeanimation-effect-to-js,kwin.upd:port-scale-effect-to-js,kwin.upd:port-dimscreen-effect-to-js,kwin.upd:auto-bordersize,kwin.upd:animation-speed,kwin.upd:desktop-grid-click-behavior,kwin.upd:no-swap-encourage,kwin.upd:make-translucency-effect-disabled-by-default,kwin.upd:remove-flip-switch-effect,kwin.upd:remove-cover-switch-effect,kwin.upd:remove-cubeslide-effect,kwin.upd:remove-xrender-backend,kwin.upd:enable-scale-effect-by-default,kwin.upd:overview-group-plugin-id,kwin.upd:animation-speed-cleanup
[Compositing]
OpenGLIsUnsafe=false
[Desktops]
Id_1=cb243d49-c96d-4eab-b49b-d86f92aa4398
Number=1
Rows=1
[NightColor]
Active=true
LatitudeAuto=28.1987
LongitudeAuto=112.9709
[TabBox]
LayoutName=com.github.vinceliuice.Layan
[TabBoxAlternative]
LayoutName=com.github.vinceliuice.Layan
[Wayland]
InputMethod[$e]=/usr/share/applications/org.fcitx.Fcitx5.desktop
VirtualKeyboardEnabled=true
[org.kde.kdecoration2]
theme=__aurorae__svg__Layan-solid
However, Idk how to dig in further.