Recently, I submitted this patch (https://phabricator.kde.org/D23082) which had a side-effect which I hadn't noticed at the time: If the user-configured maximum volume for the applet was less than 100%, it would also use this as the maximum value on the sliders for all streams.
Before this patch:
Devices tab:
Applications tab:
I have set the maximum volume in the applet settings to be 15%, and this setting is applied to all the sliders. Even the sliders which are intially at 100% get their maximum reset to 15% as soon as I interact with them. Note, for example, how even the microphone volume is capped at 15%.
After this patch:
Devices tab:
Applications tab:
The maximum volume is still set to 15% in the applet settings, but this is does not affect the maximum values of the sliders for the device or application streams.