Just tested now: using newly built Whonix VM 15.0.1.0.3, on first boot the expected 1920x1080 resolution did not launch. It appears that it is caused by the fact that Whonix VM default display name is Virtual-1 and not Virtual-0, and thus it could not launch the correct settings for Virtual-0. Replacing Virtual-0 by Virtual-1 in the skel file fixes the error (now boots into 1920x1080 mode on first boot).