When using an Assign To-Do Task action, and storing the IDs into a collection, using those IDs to query the 'Outcome' field of the workflow task results in a blank field.

Add a 'Commit pending changes' immediately after the To-Do task action.

It is necessary do this because the workflow updates the outcome column, not the user account. This causes the original value of the outcome (blank text) to be used when queried because of the way the workflow engine caches list item values. A commit action resets causes the workflow engine to get the latest value of the field.