Further evidence that this is in fact a windows bug:
If you use the keyboard media control keys to change volume, it does not change the volume on the targeted device, it changes it on the default device.
To explain this as an example:
My default device is Voicemeeter Input
In the 'App volume and device preferences' settings app, I have firefox set to output to Voicemeeter VIAO3.
If I am using firefox and use the volume keys (a knob on my logitech keyboard) then it shows (in an on-screen overlay) that the volume is being controlled.
Play/pause/skip/etc all work as expected, for example I can pause and play a youtube video.
However, the volume does not change.
Viewing the sound control panel (control.exe, click 'Sound') and viewing the properties of each sound device, and changing to the 'Levels' tab, I can see that the volume shown on screen is the volume of the default device. The volume of the device which firefox is using (VAIO 3) remains at 100% as I left it.
Windows is just bad at audio. This is why we need voicemeeter. It helps us to work around the many limitations, but it cannot do magic. This recent addition of being able to assign applications to certain devices is actually a lie. It does not only output to that device. It outputs to the default device, and then redirects to the specified device. This is why we see it in voicemeeter on both devices.
Perhaps though, it would be a good idea for voicemeeter to allow us to scroll through the list of applications shown in the channel strip. This way, when windows shows too many outputs, we can still scroll to see the ones that are hidden.... Or maybe, just maybe, Vincent is very clever and can figure out how to hide the applications which are not really outputting audio to the default device but are actually being redirected to a different device - but still there is a chance to have too many applications to see them all, so scrolling would still be a nice feature