It's happening because too much extra diesel is being injected into the engine to fire off the soot in the DPF, so the extra fuel sinks to the sump, contaminating the sump oil. Leave it too long and the sump level will rise to a point where the engine runs uncontrollably on its sump oil and can only be stopped by force-stalling it. Numerous 500 Multijets have blown up for this reason. In Mazdas, the ECU needs to be re-set following an oil service or it will continue to add excess fuel to clear the DPF whether the DPF needs it or not, so maybe the same happens with the 1.3 Multijet.