This is a temporary folder being made in the /var/lib/qubes/appvms/ despite it being a template. It creates the volatile .img in the /appvms/ folder, before fixing the template folder in /var/lib/qubes/vm-templates.
Therefore, try look at /var/lib/qubes/appvms/trim-whonix-ws/
Like you suggested, I would also try move the folder away, and see if it fixes things without breaking something. I haven't done this before, but perhaps it's best to make a quick copy backup of whonix-ws first, just to be safe.
Also, try trim another template (where trim works and succeeds), while having the window manager open at /var/lib/qubes/appvms/, you will notice the temporary folder being made, while the trimming is working.