First which MainStage plugin are you using? If it is the retrosynth then try adjusting the “CENTS” On the 2nd oscillator back closer to 12 o’clock. (Other plugins have a similar parameter). This detunes Osc 2 in relation osc 1 giving a chorusing or sometimes “phasing kind of effect”. You can also make sure there is no waveform shaping or modulation going on by adjusting that control to 12 o’clock. I don’t have the layouts in from of me but some plugins have an analog or a “fat” effect that might also be causing the pseudo phasing., Let me know if any of this worked.