It is not possible to add multiple objects to a list and join them to a string.

Only the first added object will be in the output string.

This code snippert works perfectly fine in normal groovy, but not with jenkins groovy :-/

class TemplateParameter implements Serializable {
    final String parameter
    final String value

    TemplateParameter(String parameter, String value) {
        assert parameter
        this.parameter = parameter.toUpperCase()
        assert value
        this.value=value
    }

    String toString() {
        "-p=$parameter=$value"
    }
}

node('master') {
    def branch = 'foobar'
    def revision = '1337'
    
    List<TemplateParameter> result = []
    result += new TemplateParameter('rev', revision)
    result += new TemplateParameter('branch', branch)
    println "[DEBUG] Template Parameters size: ${result.size()}"
    println "[DEBUG] Template Parameters: ${result.join(' ')}"
}

Output:

[Pipeline] {
[Pipeline] echo
[DEBUG] Template Parameters size: 2
[Pipeline] echo
[DEBUG] Template Parameters: -p=REV=1337
[Pipeline] }
[Pipeline] // node
[Pipeline] End of Pipeline

Expected:

[DEBUG] Template Parameters size: 2
[DEBUG] Template Parameters: -p=REV=1337 -p=BRANCH=foobar