To just output dimmer in gMA3 3D, select the fixture along with its sub-fixture and then press Full.
A shortcut to select fixture & sub-fixture(s) together would be to type <Fixture ID> followed by "."(dot) and then press Full.
Ex: Fixture 1. Full
Why does this happen?
At the moment, the 3D only visualises the sub-dimmers that are defined hierarchically within the geometry of the fixture.
The geometrical references that allow referring of instances to individual geometry outside the parent body of the fixture (like "Aura" in this case) are not visualised.
You can see the differences when you look at Martin Mac Aura XB [Extended].
Of course, this only affects the 3D and not the actual DMX output.
Besides, this kind of control actually helps for certain fixtures like Vari-Lite VL5LED Wash, where the Main Dimmer control should not also control the LED Blades.
Regardless, there are plans to improve this handling better than what we had previously.
I hope that answers your question