SUMMARY
Issuing a "Mute" shortcut (Shortcuts > System Services > Audio Volume > Mute) mutes all output devices. However, subsequently issuing a "Increase Volume" or "Decrease Volume" shortcut only unmutes and adjusts the volume of the default device.

This behavior is unintuitive—especially in setups with multiple virtual output devices (e.g., EasyEffects sink, GPU Screen Recorder sinks)—as it leaves secondary outputs muted indefinitely.

STEPS TO REPRODUCE

1. Ensure multiple output devices are present and unmuted.
2. Trigger the "Mute" shortcut.
3. Trigger the "Increase Volume" or "Decrease Volume" shortcut.

OBSERVED RESULT
All output devices are muted by the "Mute" shortcut.
When a volume shortcut is triggered, only the default output is unmuted and adjusted; all other outputs remain muted.
Toggling the "Mute" shortcut again does not restore the mute state for secondary devices—they remain muted until manually unmuted.

EXPECTED RESULT
One of the following would be expected:
- "Mute" mutes only the default device; volume shortcuts affect only the default device.
OR
- "Mute" mutes all output devices; volume shortcuts affect all output devices.
OR
- A separate shortcut is available to mute/unmute just the default device. (I think this would be the best option.)

SOFTWARE/OS VERSIONS
Operating System: EndeavourOS
KDE Plasma Version: 6.3.5
KDE Frameworks Version: 6.13.0
Qt Version: 6.9.0

ADDITIONAL INFORMATION
This behavior appears to have been addressed previously in the PulseAudio applet:
https://invent.kde.org/plasma/plasma-pa/-/commit/3538c92ebb6a8d446dedb3c7324d629d8904bbed
However, the fix may not apply under PipeWire, and this inconsistency still occurs in current setups using PipeWire.