I tried to run using PySide2 5.12.1 a program I wrote that runs without any problems with <=5.12.0, and now I get a "segmentation fault" at the very beginning of the execution. According to faulthandler it happens when some submodules of matplotlib are loaded. Apparently, the problem is solved if I load matplotlib.pyplot (or the backends) before the PySide2 components, but in realty the crash is delayed when some functions involving matplotlib are executed.
I don't think it is a matplotlib problem, as I use the same version 3.0.2 but only fails when PySide2 5.12.1 is loaded.
It also seems that the failure does not depend on the matplotlib backend that is used (it fails either with Qt5Agg and Agg).