The following function demonstrates the problem:

Object func1() {
    Object m = [:];
    m.with { x = 1 };
    return m;
}
assert func1() == [ : ]; // wrong, should be [x:1]

This is supposed run the "x=1" in the context of m. However, func1() returns an empty map.

If func is marked @NonCPS, it returns map [x:1].

@NonCPS
Object func2() {
    Object m = [:];
    m.with { x = 1 };
    return m;
}
assert func2() == [ x:1 ];

Running these in a Jenkinsfile.  When run in the script console, the @NonCPS is not needed (behaves as expected).
My expectation is that func1 should work without needing a @NonCPS annotation.