Resolution: Won't Fix
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.
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.