
did you pressure test the system.
i would replace the stat with a gen mazda, and split off as many pipes as you can to check flows, especially the front heater matrix and the rear matrix, during bleeding and flushing the system you may have disturbed the crud and sediment around the system, this usually settles/collects in the lowest points of the system or lodges in the smallest diameter apertures,(front and rear heaters). then doing a vac fill the crud restricts water entry into the system, as the water is sucked in via another route,this may encourage airlocks.
the vac fill method only works on clean unblocked systems and will not take into account faults/blockages, as it will always take the path of least resistance.