Why am I only getting stereo audio passing through OMNIBUS, when playing Dolby Atmos Conent from Apple Music?

Make sure you have Dolby Atmos ‘always on’ in Apple Music’s preferences, this will output the discrete 7.1.4 channels into OMNIBUS for routing.

If the incorrect channels are passing through OMNIBUS, go to audio midi setup and check the speaker configuration for the omnibus drivers are set to 7.1.4 and that the channel layout is set to 1-12.