Understandable that you do not need help fixing, but you may be able to help diagnose or lead to a fix.
I do not use Gnome and have not for a while, now. But, this effect seems similar to that of Dynamic Panel Transparency, a Gnome-Extension that is for gtk 3.22 and up. (XFCE has a similar version: xfce4-panel-auto-opaque.)
Whether or not Zorin uses either - I have no idea. I looked but did not find anything. In my mind, it would make sense for a distro to use similar features on two variants for uniformity. It is what I would have done.
Gnome users may be able to check for it using dconf-editor under org/gnome.shell/extensions. At least, I think that is the path.