How do you see it is mono? Is it really a channel missing in the recording?
after the big sur update with MainStage 3.5.1. my recordings sound really weird. As if my headphone is not wel plugged in , or low res. It does have two channels though in the recording. When I play back, sound is terrific, as usual. CPU load is below 20% or so, not a glitch whatsoever. However the recording sounds really bad and it has glitches: for like 0.1second the left channel has all 'zeros' , a flatline, and then the signal continues. Occasionally it is on both channels. It will do this every 3 or 4 seconds. All in all, my MainStage rig became unusable for recording a song in my homestudio. Live playing will still work.
I use a Focusrite 4i2 audio interface, but the problem also occurs when I disconnect it and use the internal sound only. It gives the same result. It sounds great through the headphone while playing, but the recording sounds muffled and it has glitches.
I am really stuck on this. One of my favourite pastimes is not possible in this way