xfce4-panel: some notification area icons change with message popup, and don't revert (panel 4.15 (4.16pre1))

when some applications issue a popup notification, their icon in the system tray/notification area changes to an info icon (see attached image). clearing the notification does not revert the icon to its' previous state.

FWIW, this behavior is not observed in the 4.14 panels.