Behaviour of Master and Capture sliders

[ultrarunner2018]

Hi;
I just installed the Audio Injector Stereo Soundcard onto my pi 3 b+ (stretch).
I have noticed a bit of odd behaviour with the mute button in Playback>Master or the recording button in Capture.
If the button is pressed to mute or stop recording, then pressed again to restart the audio, nothing happens until you move the slider a bit.

I suspect a bug in the pi (stretch) Audio Device Settings code and not with the Audio Injector driver, but I can't be sure, since no other audio device I use (I have a Behringer UMC202HD that I can connect to the pi via USB) uses these controls.

Any ideas?

Thanks

Ultrarunner