xfce4-notify Opacity setting affects all windows after reboot
When Opacity is changed in the Appearance section of the Notifications settings window, then following a reboot the chosen opacity will get applied to all windows instead of just the notification popups, regardless of whatever opacity settings you otherwise had in XFCE. Setting Notification opacity to 100% and then rebooting fixed the problem.
Problem occurred on Manjaro Linux kernel 6.0.19, with XFCE 4.18 and xfce4-notifyd 0.8.0.