Cascading Select List object only corresponds to first selection

Cascading selects behaviours only apply to the first part of the cascading select. This is reproduced while grabbing value, while setting the error, and to any interaction with the cascading select. Any of the before mentioned only get applied to the first select, not to both of them. The field behaviour doesn't get triggered when the second field is updated as well.

Steps to reproduce:

1. Create a Cascading Select List custom field
2. Create a Behaviour that uses it

3. In the Behaviour, have a server side script that grabs the Cascading Select List and outputs its value to an error like so:

def source = getFieldById("customfield_10300")

source.setError(source.getValue() as String)

4. Apply a mapping to a test project and issue type of your choice

5. Apply the custom field to the edit screen for the project

6. Create an issue in that project with the corresponding issue type

7. Edit the issue and modify the custom fields values

Notice that only the parent selection is getting the error and only it's value is being output in the message. 

Notice the behaviour only gets triggered when interacting with the first part of the select, not the second.