Is there a solution to this? When a user goes in to the edit the task form, every field - except the list lookup dropdowns and checkbox button values and attachments - inherits existing data.
This problem does not occur if editing the form via the SharePoint list itself.
It seems this issue of list lookups is particularly described in 3 other threads:
List Lookup to inherit existing column data
How to Populate the Field values from the Nintex form to Task Form in Nintex 2013
How to keep lookup item from form all the way through workflow