Hi adriaan36,
(1) How do you see it is mono? Is it really a channel missing in the recording?
=> Musically, my ears heard it. Technically, Audacity app let me see that the left and right channels are identical...
(2) Did you fall this as solved?
=> Yes this problem is solved because Mainstage 3.4.4 is enough for my music. You can use the previous version 3.5.0 if you have a TimeMachine backup.
(3) If this flag prevents apple to look into this post, please remove that flag!. Downgrading is not a solution really...
=> I warned Apple about this bug by reporting an official incident: hope this problem is solved in the next version Mainstage 3.5.2