I wonder if it has to do with casing.
You have a rule that sets the Supervisor field.
If there is not a match in the data source for the drop down, you'll see your entry that you set via rules, and the available drop down choices.
You may want to drag the secondary data source you are using for the drop down onto your form. Drag the Supervisor node onto your form too, not as a drop down, so you can see the actual value. Then test it in the browser - that should give you a good idea of what is happening.