Each invoice item in the mobile API now includes an expense_uuid when the item bills an expense. A draft saved with the same value keeps the expense linked. The mobile app cannot add expenses to an invoice; an app version that does not send expense_uuid back removes the link from any expense items when it saves the draft.
The mobile invoice API keeps expense links
Invoice items returned to the mobile app include the expense they bill, and saving a draft with that value keeps the link.