The theme overrides were not working, so the issue is likely in how the Qt5 template layer was built/integrated in the universe MATE build (Qt6ct seems unaffected, but will not help with code built on Qt5). i.e. the applications may not crash, but will look garbled and exhibit undefined GUI behaviors on legacy desktops. Overriding the themes and other documented workarounds will only suppress standard warnings, but have not had any impact on the core Qt5 issues.

For an LTS OS intended to be around for 12 years, I am not sure things have turned out well in user space and Qt5.

Hopefully, this saves someone a few hours of chasing this external issue in their own projects. =3