Created attachment 8380
Attaching a screenshot.
Anyone can validate, please?
It uses the icon gtk-fullscreen (https://git.xfce.org/xfce/xfwm4/tree/src/menu.c#n50).
Make sure your icon theme has one.
Same here. Could you show me how it looks for you?
Tried different themes: https://drive.google.com/file/d/1tgTDP2mxG8_OawMbH9Y5tfuuElosWI_N/view
Created attachment 8385
Looks good here with Moka (screenshot), Adwaita, Elementary and Faenza.
Let's close this as invalid. User error on my system which I will fix somehow.