I guess it depends on what you have you Master set to in the Output menu; Master mono or Master stereo
Did you mean the Monitor Output? In other words, ensure that "Monitor Stereo" has not accidentally been selected in the Ouput / Monitor settings. The default setting on all the classic KPA models (not sure about the Stage) is Monitor Mono..
I don't think the Master Output settings have any effect, or are relevant to how the Kemper Kone (which is always connected to the Monitor/Speaker Output) reproduces a stereo playback signal connected to the Aux In.