SUMMARY
When you change the default pulseudio sink from the one of your sound card to another (maybe the equalizer or PulseEffetcs), pulseaudio won't control the volume of the default sink when audio is playing.
It only controls the default sink when no audio is playing.

STEPS TO REPRODUCE
1. You have a pulseaudio sink, let's call it "Stereo Internal Audio", linked to your sound card and it's the default after a fresh install.
2. Make a new pulseaudio sink, i.e. using PulseEffects, and let's make it the default sink.
3. Change the volume through keyboard key and PulseEffects default sink is affected correctly.
4. Let's start a song or a video file and you make sure that the default PulseEffects sink is used by the application.
5. Change the volume and you will see that PulseEffetcs sink IS NOT affected by plasma-pa. Volume is changed on the sound card sink: Stereo Internal Audio. You can also notice that nothing is changed in pulseaudio configuration: PulseEffects is still the default sink.
6. Stop the reproduction, close the mediaplayer application and change the volume again. PulseEffects is affected.

OBSERVED RESULT
plasma-pa does not respect the control on the default sink when an audio stream is reproduced.

EXPECTED RESULT
plasma-pa should always change the volume on the default sink, or at least it should let you choose the preferred sink to control even if it's not the default in the pulseaudio config, just like it's done in kmix.

SOFTWARE VERSIONS
(available in About System)
KDE Plasma Version: 5.14.3
KDE Frameworks Version: 5.51.0
Qt Version: 5.11.2

ADDITIONAL INFORMATION
kmix is another volume manager and it lets you choose one sink to control and volume control always affects that sink even if it's not the default sink. While the behavior in plasma-pa is clearly a bug because considering that: 

- It does not let you choose a preferred sink to control;
- It lets you switch the default pulseaudio sink changing the pulseaudio configuration and the sink to be used by all applications

When you set a default sink different from the one associated to the sound card and reproduce something, it won't control the volume on default sink.