Relationship field seems somewhat limited. One thing (for which there are other posts for) is that recurring tasks using the create new task method don't inherit/duplicate the relationship.

Additionally, another thing which has made it difficult for us is that if a list is filtered for 1 particular field value say in a dropdown field, and then we add new tasks using that view, the new tasks automatically take on that field value (which is great) - but we just noticed today that it doesn't work for relationship fields. Eg even if my list is filtered for 1 particular relationship field value, and then I add a new task, the relationship is not created. This is really painful for us. Hope this can be put in place.