On systems with a blacklisted GPU / driver (through the integrated blacklist json file) the value of QT_ANGLE_PLATFORM is not used. Instead the setting found in the json file is used.
I tried to fix https://bugreports.qt.io/browse/QTBUG-42842 by setting QT_ANGLE_PLATFORM=d3d9, but it had no effect on the system, because a blacklisted Intel HD3000 GPU is found and "features" = "disable_desktopgl" is used (clearly Angle was used in Qt, though a desktop OpenGL 3.0 context was available on the system).
Using an external json file with "features" = "disable_desktopgl", "disable_d3d11" and setting its path using QT_OPENGL_BUGLIST does fix the problem as expected...

I expected QT_ANGLE_PLATFORM to override the angle backend in the "features" section. If you do not decide to change this it would be nice if the interaction was documented.