When using volume media control keys of shortcut keys, volume OSD (on screen display) is not visible if the default panel is set to auto-hide. However, if the panel is made visible using the mouse, then volume OSD works. Also, if one scrolls the volume up/down using the mouse wheel, volume OSD works. If the default panel is set to always visible, volume OSD works. Reproducible: Always
The first sentence should read: When using volume media control keys _or_ shortcut keys, volume OSD (on screen display) is not visible if the default panel is set to auto-hide.
Can confirm.
can reproduce with plasma-5.8.2, marking confirmed
*** This bug has been marked as a duplicate of bug 370433 ***