this is new in 6.9
using external audio device (not built in)
qt reports "format unsupported" for 16bit stereo float, which is a fundamental format type
when asking device for its preferred format, then asking if it supports that format, device still says it is not supported, see screen shots

update:

the "Audio Devices" example project that ships with Qt confirms that for external devices, only 48000 Hz is supported. Previously, 44100-96000 Hz were supported.