Video and music volume is permanently decreased after notification & notification dismissal

I am watching a video or listening to music.

Notification received.

Volume of video/music is decreased (supposedly just for a few seconds or until notification dismissed)

Dismiss notification

Volume at low level still.

Volume bar still shows level at previous volume setting

Even worse when I am watching at high volume as then I cannot even increase to listen at normal volume - as the bar is already at max.

After ten minutes, when nothing special is happening (no specific notifications etc) then the volume MAY increase again (maybe it won't maybe it will)

This despite only a single, or multiple notificiations arrive after the initial first one

Really annoying as I listen to lots on my iphone