I neon me and David R tracked it down to some optimization probably with the dh_dwz tool from Debian packaging tools, this is the "DWARF optimization and duplicate removal tool" part of the toolchain.
And with the discussion here Christoph has yesterday replaced the use of qt_add_big_resources() with qt_add_resources() which also makes the problem go away. https://invent.kde.org/frameworks/breeze-icons/-/commit/fa44b11bc2b36d5c3cfc5c3403ea75b2fff57253 So beware use of qt_add_big_resources() it seems to be fragile to optimising. Jonathan