Mainstage 3: Output bus not being added when required

I have just upgraded to Mainstage 3.6.6 but have found an issue. I am using a Behringer XR18 mixer for output for Mainstage which gives me 16 output channels. However, when I select (say) output 11-12 in one of my patch buses, it doesn't add an output bus for 11-12 in the output bus section and no sound is sent to channels 11-12 on the Behringer. I have reverted back to version 4.2.3 and it is working perfectly. Am I missing something? Any help would be greatly appreciated. Thanks, Darren.