User Story map should support epics that span columns

I created a user story map with multiple columns, each for one activity in my flow. Now I want to pull features from multiple columns into an epic and move into JIRA. However, as soon as I assign to a master feature, the user story map pulls the features into one column under that master feature, since master features can only occupy the second row.

i'd like a way to have a master feature that spans multiple columns in the user story map. Assigning features to a master feature should not pull them out of their columns in the story map. Teams organize epics in different ways, so saying an epic should be one user activity in the flow seems limiting.