What's happening:
When editing an Action Trigger that has been set up to create a Task and it has previously been enabled, it is possible that the "Assigned To" field and "Who Receives the Action" fields will not be able to be changed in certain situations. After saving the settings they will default back to what they were set to previously.
If a user needs to change who is listed in the Who Receives the Action section of an Action Trigger, they will first need to change who is listed in the Assigned To field.
After changing the Assigned To field to another option, the user will be able to remove the original option from the Who Receives the Action section.
However, this is not possible if a user is attempting to change the Assigned To field to Queued (by selecting the blank option at the top of the dropdown) or wishing to have the Who Receives the Action section to only have Worker Category listed.
If an Action Trigger needs to go to a Worker Category only and the associated task needs to be unassigned and go to the Task Queue, the Action Trigger will need to be remade with these settings and the original Trigger disabled.