When you create a variable composition mixture, at any given control volume the sum of the components must be exactly one. To enforce this, one of the components must act as the constraint (i.e. whatever is left over in the control volume must be that component in order for the mass fractions to sum to one). In your case, you're showing that the constraint component is filling most of the volume when using Volume Rendering. This implies one of two things:
1.) The mass flow rate of the constraint into the model component is much larger than that of the other components.
2.) The case isn't converged such that mass imbalances are low. Depending on how the case was initialized, the model might have contained all or mostly the constraint component at the beginning of the simulation. If the simulation was stopped before the model was converged and the flow imbalances were minimal, one might see mostly the constraint component in the model.
You can also verify the volume rendering by creating planes in the model and coloring by the component mass fraction.
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