See https://bugs.kde.org/show_bug.cgi?id=487374 for the original bug report. Now its time to find the root cause
Any chance libplasma can cause this?
If it affects apps at all, then no. Even if it's Plasma only, I doubt it. All these icons are drawn using Kirigami.Icon, which is in Kirigami, not Libplasma.
Can't reproduce that with either master or the 6.2 packages that NixOS ships.
If somebody who can reproduce that and do a package build in the way it happens, one could try if e.g. https://invent.kde.org/frameworks/breeze-icons/-/blob/master/src/lib/CMakeLists.txt?ref_type=heads Using here qt_add_resources instead of qt_add_big_resources would help, as people said that might be an issue.
https://invent.kde.org/frameworks/breeze-icons/-/merge_requests/379
Yes it was qt_add_big_resources being optimised. Use of qt_add_resources fixes it. And stopping use of dh_dwz debhelper script also seemed to stop the problem.
Nice!