Bug: Audio Correction -> Balance does switch off left channel only

STEPS TO REPRODUCE

1. Add a clip or an audio file onto the timeline
2. add effect: Audio Correction -> Balance

OBSERVED RESULT

Effect does switch of the left channel only. Moving the slider has no influence.

EXPECTED RESULT

Moving the slider should balance between left and right audio channel

see here: https://forum.kde.org/viewtopic.php?f=271&t=161797
and here: https://bugs.kde.org/show_bug.cgi?id=410449