Question regarding multiplayer factions compatibility

[Arizona Ranger]

Hi, I recently bough the Tau faction and my friend yesterday bought the Imperial Guard, but when we try to play multiplayer the only matches we see are those hosted by factions we both own (for me Marines, Tyranids and Tau; for him Guard, Marines and Tyranids). We can play just fine if one of us picks a faction we both own but we can't seem to be able to find each other's matches if we want to play Tau vs Guard.