Entry saving in task fails due to required assets/matrix fields being read as blank, even through they are not
This is due to the way Craft manages custom fields at the moment (but has been “fixed” for Craft 3). Whenever you save an element, you need to make sure all of the field values are in the same format they would be if they had come from an Edit Entry form submission. Here’s another post that describes what that looks like: How to save a matrix content of a new entry in my plugin?