Using jenkins pipeline, I have found that an environment property that takes variable substitution (see example below) doesn't get it's backslashes correctly reduced. I would have expected that both keys below start with "a\b". Instead, AAA_Key1 becomes "a
b 0", while AAA_Key2 becomes "a\b".

Example script

pipeline {
    agent any
    stages {
        stage('Build') {
            environment {
                AAA_Key1 = "a\\b ${EXECUTOR_NUMBER}"
                AAA_Key2 = "a\\b"
            }
            steps {
                bat 'set'
            }
        }
    }
}

Current Output

c:\w\Pipeline Experimental\default>set
AAA_Key1=a\\b 0
AAA_Key2=a\b

Current workaround is to use withEnv directly, which doesn't appear to suffer from this issue