Looks like its the Windows device properties. Default setting is OK, but the problem looks to be there. Gone into Sounds > Playback > Configure and the settings don't seem to be "sticking" within Windows. I apply a config, save it, and when I go back it has reverted. Also seems to be happening on a different machine, ie. the one I'm typing on. Might not be recent, but looks like some of the settings that used to be in the old Sounds control panel and also appearing under Settings.
One specific query though, which might be a red herring. The HDMI audio output device (which is default) is obviously picking up that there is projector attached to the receiver, and is setting the device name to Optoma UHD. That projector only has stereo speakers, whereas the Sony receiver that the signal is passing through has all the audio goodies including Atmos. If I try to configure the output to Dolby Atmos, it reverts to Stereo. I don't suppose that Windows is limiting the options available based on the characteristics of the projector?