If there is a class in a library as well as some vars and the class's constructor tries to a var from the same library the new()'s result will be the var's result and not a class of the requested type.

Sample code:

src/com/test/X.groovy:

package com.test
public final class X {
    public final String result
    public X(def steps) {
        if (steps.y())
            result = 'y returned TRUE'
       else
            result = 'y returned FALSE'
    }
}

vars/y.groovy:

def call() {
    return false
}

Jenkinsfile:

lib = library 'testLib'
x = lib.com.test.X(this)
echo "x = $x"
echo "result = ${x.result}"

If testLib is set to contain the above code and the Jenkinsfile executed, then it will print:

x = false

and throw an exception:

org.jenkinsci.plugins.scriptsecurity.sandbox.RejectedAccessException: No such field found: field java.lang.Boolean result

Calling into vars from other class methods seems to work fine.