5.12.0 -> 5.12.1 Regression: kde/plasma breaks when using oxygen theme [bisected to 9d90c0edac91b35ec96646fd3e6cdd339639ca79]

I use archlinux 64 bits, kde-plasma as desktop management,

since Qt5 5.12.1 I notice that kde-plasma is broken when the oxygen theme is used,

the desktop displays but the mouse doesn't move, and the harddisk led blink for a long time, the entire system seems frozen (but I can open a terminal with ctrl-alt-f2),

if I downgrade qt5 to the previous version (5.12.0) then the oxygen theme is usable, no bugs, no hangs in kde-plasma,

and If I use the default theme (breeze) then no problem with the last version of qt5 libs (5.12.1-1)

so there is an incompatibility between oxygen 5.14.5 and qt5 5.12.1-1

STEPS TO REPRODUCE
1. install kde plasma 5.14.5
2. use the oxygen theme 5.14.5
3. update to the last qt5 lib release version (5.12.1-1)
4. reboot your system (important)
5. at start you will notice that kde-plasma hangs when desktop is displayed (mouse frozen)

I use radeon HD4650 pci-e graphic card, with the open source driver (radeon), openGL2.0 acceleration in plasma settings for the compositer

some archlinux users have reported that with nvidia driver (even nouveau driver) they experience also hangs/crash, even with breeze theme,

I didn't manage to find useful infos in logs when the bug occurs, it seems a silent crash/hang