I think so, yes: as the mixture fractions change, the average molecular weight will change, and therefore also the density.

I think the issue might be related to the fact that for mole fractions, I cannot plot contours of boundary values. See screenshots below. 

Image 1: boundary values of mass fractions: constant mass fractions

Image 2: cell values of mass fractions, not constant anymore!

Image 3: mole fractions of cell values, not constant, and cannot select boundary values

Is there a way to plot the actual face values and not cell values for variables such as mole fraction or relative humidity? I always assumed that on boundaries, the cell values would be interpolated to face values, but that seems not to be the case in contour plots. 

The strange thing is: if I define the mole fraction using an expression (image below), I also cannot select "boundary values". However, when I make the contour plot, Fluent does give a constant value, so that shows what I expected on the faces... 

Image 4: mole fractions from expression, gives constant values!