Missing titlebar and broken minimize/maximize/close buttons

Ensure that the issue is present with the latest Arc-theme release, or latest git snapshot: Done
Test whether the issue also appears with default themes (e.g. Adwaita for GTK and GNOME): it doesn't (see here)
Make sure you have built the correct theme versions for GTK 3, GNOME Shell and Cinnamon themes.
Make sure the issue persists between restarts: it persists (even without saving session after restart)
For GTK 3 related issues, try clearing your user's ~/.config/gtk-3.0/gtk.css, if it exists: (it doesn't exist)

Distribution and version: Fedora 34 Xfce spin
Versions for the affected applications/desktops/tookits: Xfce 4.16 / xfwm 4.16.1
Installation method (package or compiled manually): package
Arc-theme version: 20210412

Screenshot:

jnsh / arc-theme