I may just be shooting into the wind here but my suspicion is that the
whonix-ws-15 template does not ship with all of the necessary Qubes
packages for Qubes functions to work as expected.
Like in the case of minimal templates,
https://www.qubes-os.org/doc/templates/minimal/, you have to install
packages according to your needs for specific functions and or
functionality. I don't think the whonix-ws-15 template necessarily has
what you need for USB device passthrough to work properly.