Something ayatana-indicator-application is doing appears to be preventing other notification plugins such as the xfce4-panel "Status Tray Plugin" from displaying all application icons in the notification area.
Prior to upgrading to 23.04 I could see Steam, Clementine, and Discord icons. After upgrading from 22.10 to 23.04, Steam, Clementine, and Discord icons disappeared. I could not figure out why until I ran across the following bug submitted to xfce4-panel: https://gitlab.xfce.org/xfce/xfce4-panel/-/issues/599#note_61443 The resolution and fix to get application icons to appear was to remove ayatana-indicator-application and ayatana-indicator-common. I also removed ayatana-indicator-application and ayatana-indicator-common, and my Steam, Clementine, and Discord application icons reappeared and have remained visible.
Status changed to 'Confirmed' because the bug affects multiple users.