So this was working fine before Pipeline Model Definition version 1.1.
==============================
#!/usr/bin/env groovy
 
def SKIP_BUILD = false
 
pipeline {
    stages {
        stage('build') {
            when {
                expression {
                    return !SKIP_BUILD
                }
            }
            steps {
               echo "Do build work here."
                **                
            }
        }
 
    }
 
}
================================
 
In version 1.0.1 the variable "SKIP_BUILD" was bound correctly and the expression could properly evaluate it.
 
As of version 1.1 (and still in 1.1.4) this results in 
groovy.lang.MissingPropertyException: No such property: SKIP_BUILD for class: groovy.lang.Binding at groovy.lang.Binding.getVariable(Binding.java:63) Same issue if I was trying to use a function in an expression, it used to work now it does not bind. Both bind fine outside of the expression block.
 
This is blocking me from upgrading Pipeline plugins for Jenkins