It appears that when D.Va deletes a Moira's Biotic Orb with Defense Matrix, she gains ownership of it for a split moment. Any healing that the orb is doing at the time of it being deleted is credited to D.Va.
A similar, but rarer bug has been around for a long time and is known to affect Junkrat as well. I am not sure about the causes of that one, but it seems related to getting healed while calling a new MEKA.