SUMMARY
After a set of updates on 27/02, which weirdly had a lot of applications request a system reboot, both on KDE Neon Git Stable and Unstable, some Breeze icons (as can be seen from the menu) got broken, namely:

Applications: System Settings, Dolphin, Kate, Kolf, Kpat, Gwenview, VLC, Kleopatra, Ktimer, Kgpg, Kwrite.

Menu sections: Languages, Mathematics, Graphics, Settings, System.

Dolphin's icon disappeared from Task Manager as well.

Active Window Control does not show its button icons (by this I mean the window decorations for minimize, maximize, close and pin to all desktops), but they exist both on the panel, latte-dock (used as example on attachments), and on its settings window.

This does not change whether one uses Breeze, Breeze Dark or another theme or color set.

If one changes icon theme to Oxygen, icons are present, including those from Task Manager, but not those from Active Window Control.

Please see attachments.
Images 2, 3 and 4 show the positions where clicks act as intended on Active Window Control, so it's certain the buttons are there.
On Image 5 I click and hold as per usual reorganization of buttons on Active Window Control settings, acting as intended. 


STEPS TO REPRODUCE
1. Update KDE Neon Git Stable or Unstable to latest
2. Reboot as prompted


OBSERVED RESULT
Absence of some icons;
No buttons on Active Window Control.

EXPECTED RESULT
All Breeze icons should be present;
Buttons should show on Active Window Control.


SOFTWARE/OS VERSIONS
Operating System: KDE neon Developer Edition (Git Unstable, but also occurred on another machine with Git Stable)
KDE Plasma Version: 5.15.80
KDE Frameworks Version: 5.56.0
Qt Version: 5.12.0
Kernel Version: 4.15.0-45-generic
OS Type: 64-bit


ADDITIONAL INFORMATION

This seems to either be i) a Neon packaging problem, since it broke not only icons but also window decorations, or ii) a Breeze icons problem, if the window decoration takes its min/max/close/pin icons from the Breeze stack.

If also tested on a different rolling-release distro, this should confirm whether it's a packaging problem or not, I believe.

I have a list of the packages that requested reboot as well, if needed. I do not have the full list of applications updated.