Independent Device and Application volume controls are confusing and limiting. - System Tray Audio and/or keyboard volume control adjusts only the Device Output and not the Application Playback volume. - Applications (Amarok, Clementine, Cantata/MPD, Chrome, etc.) adjust only the Application Playback volume and not the Device volume. The problem... KDE system audio (for example) Outputs to the Device, which is generally set to a comfortably low volume. Applications must then be set to 100% just to play at that same volume. However, client controllers such as KDEConnect control only the Application Playback volume, which is already at 100%. Thus, one cannot use the application to increase the Application volume to a preferred level, which is being limited by the lower Device volume. Reversing the volumes gives greater control to applications, but blasts system audio. Why not a single volume control, which is what I suspect the end-users truly expect and assume.
Why not set the Device volume to the maximum volume you need and control application volumes individually?
The problem with that is that the System audio events (e.g. KDE Login/Logout, etc) are then at full volume.
But you can control the Volume of notification sounds separately. kcmshell5 pulseaudio > Applications tab
So you can. Although, for whatever reason, that seems to have no actual effect on notification volume. Rather, the notification volume can only be changed (on my system) by adjusting the Device volume. In fact, even muting the notification settings has no effect.
That seems like a separate bug. If you're still experiencing it in Plasma 5.17 or 5.18 (I cannot reproduce it), please file a new bug report to track that. Thanks!