Adwaita theme is broken since 4.15.2
How to replicate on Xubuntu 21.04 (you can use a live session):
- Go to appearance and change theme to Adwaita-dark
- Logout session and login
- Right click on panel, then click on Panel -> Panel Preferences... -> Appearance tab -> move the last slide (Opacity Leave)
Nothing will happen. However using default Xubuntu theme or simply reverting xfwm4 to 4.15.1 fixes the issue.