Anyone is able to edit a custom field that is restricted by behaviours to Read only or Hidden. This happens when you first open the JIRA Issue or after a refresh. When you change another field on the ticket it will start working again but all it takes is a refresh to give you the ability to edit the custom field again.
JIRA Server: 7.1.10
ScriptRunner for JIRA: 5.0.4
Here is the JIRA Issue:

Here is the Behaviour Mapping:

Here is the Behaviour Settings:
