I'm not sure what's wrong here. I've added a behaviour which hides summary and sets the request type name of the chosen request type:
The thing is that this works for agents with a jira license, but not for customers without a jira license.
Not sure which Scriptrunner version you're using but it could be related to this bug here: https://productsupport.adaptavist.com/browse/SRJIRA-4876
It was fixed in Scriptrunner version 6.15.0.
Setting the Summary can be performed by JSM native functionality and copying field values can be performed by a post function. This raises the question of why you are using a Behaviour.
To use native functionality:
It seems to me that you are going against Atlassian's intentions for the Summary field. Please consider why it is the only field that is always required and how that is playing into your problem. The best solution is likely not technical, but changing a real world process or behavior.
You can investigate the portal section of your script (referenced in https://scriptrunner.adaptavist.com/5.5.9/jira/CustomisingUI.html) if you would like to continue with the technical solution. Cheers!