Tested with GPU "AMD Radeon R9 200 Series" and driver version 18.104.22.168 on Windows 7.
The driver does not return a context with a QSurfaceFormat::CompatibilityProfile when QSurfaceFormat::DeprecatedFunctions are not requested as well, i.e. when WGL_CONTEXT_FORWARD_COMPATIBLE_BIT_ARB is set.
If the issue exists on a specific system it can be reproduced with the attached sample code. It is important to request a context with a version >= 0x0300 (see qwindowsglcontext.cpp, ll. 503ff).
According to the OpenGL wiki, one should "should never use the forward compatibility bit" (better is probably "hardly ever", see  at the very bottom).
QGLFormat does not support DeprecatedFunctions, only QSurfaceFormat does. The default behavior of QSurfaceFormat is to not set the DeprecatedFunctions flag, which is contrary to what is suggested in . As a consequence, it is currently not possible to request/configure a default context via QGLFormat::setDefaultFormat which supports OpenGL version >= 0x0300 and the CompatibilityProfile on some graphics cards.