Rob
Ansys Employee

/define/models/species/inlet-diffusion?    Default is yes, if the flow rate is very low diffusion sources can override what's going on in the domain if you're not careful with the settings. 

The theory is covered here  https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_th/InterfaceSpeciesMassTransfer-3A72B96D.html   What are the operating and domain pressures?